I'm thinking - NL already goes a long way to fight unfair level designs. But there's likely room for improvement here.

My intention here is not to outright disable such levels. There are several reasons for this - the risk of false positives, preservation of historical content, some might even have very rare legitimate use cases.

Instead, I'd like any solutions to take one of three forms:
a) An explicit warning message to the player. I'd kind of like this to be an intrusive, unmissable one (a popup of some kind); but it could also be something a bit less in-your-face like a "WARNING: May contain unfair content" text on the preview screen.
b) A conditional, or just always-shown, helper of some kind. For example, perhaps a big "EXIT" with an arrow appears above / below the exit at the start of the level, preventing it from hiding behind terrain even using workarounds like "fill the entire level with an updraft trigger area". This could even be more generic, just a "WARNING" sign that alerts the player that something dodgy is going on in a certain area, rather than specific graphics for each situation.
c) Additional clarity in Clear Physics Mode. This would likely be in addition to, not instead of, one of the above.

Regarding the situations this should detect, of course, some are more realistic than others to detect in practice, but some situations I'd consider:
- Objects (other than purely-decorative ones) hidden behind terrain
- "Only on terrain" objects that don't overlap any terrain - although my more-instinctive thought here is that only-on-terrain objects should be non-functional in the first place
- Multiple overlapping windows, if this affects the state of spawned lemmings or the spawn order
- Multiple overlapping pickup skills (even if they are the same skill, the overlap will affect how many)
- Objects that have functions, but blank graphics (and thus end up invisible)
- Identical in appearance steel and non-steel pieces (this would only be able to go as far as detecting them when used in the same level, and would be very limited as to how much it can detect those that are merely "similar")
- Perhaps any piece explicitly marked as "UNFAIR" in the NXMO / NXMT file, for those who are choosing to make such packs while being honest and open about it (or as a tag that can be added in the styles download to pieces that have been identified as unfair but are not detected by NL's automated tests)

If false positives / flagging of legitimate use cases become too much of a concern, this could perhaps be combined with some kind of centrally-maintained whitelist of levels that are confirmed to be fair. On the other hand, my sympathy for historical content only goes as far as "don't outright prevent it being played" - I am 100% okay with such levels being flagged with a warning, my own included in a couple of cases where I've made designs that don't stand up to today's standards.

the risk of false positives

What does "false positives" mean, particularly in this context?

- Objects (other than purely-decorative ones) hidden behind terrain

Maybe NL could be configured to display a CPM-mode version of the object graphic when it's completely hidden behind terrain. So, a bright pink shape would appear.

- Multiple overlapping windows, if this affects the state of spawned lemmings or the spawn order

I've suggested a possible solution to this here. TL;DR - add A, B, C, D etc... helper icons above entrance hatches (as there's currently no CPM way to determine order anyway), and in cases where they overlap configure NL to display the helper letters next to each other rather than themselves overlapping.

"May contain unfair content" doesn't actually give the player the information they need to interpret what is on the level

I agree, and I too wouldn't like to have popups or pre-screens for such levels.

I understand the idea, though, and I myself have previously suggested that packs could be labelled as [HIE], meaning that they contain hidden and/or invisible elements.

Maybe, in the case of popups, if these were implemented in a more specific way (e.g. "this level contains a hidden exit" or "this level contains overlapping teleporters"), and be made optional so that users can disable these popups if they aren't bothered about hidden or invisible stuff, then this would satisfy just about everyone.

Thinking about it, this might even be a decent enough way to settle the whole thing once and for all!

What really matters is that we have a community consensus that we should try to be fair in our level designs going forward, and are willing to listen to each other and make changes in response to feedback.

+1 for this!

Maybe there should be a community-agreed list of things that should and shouldn't feature in levels, grouped as:

Acceptable: most people in the community agree that these features are acceptable.
Controversial: there is divided opinion on these, so expect that some players will not take kindly to use of these features, but others may enjoy these features.
Unacceptable: most people in the community agree that these features are unacceptable (this would mostly be features that are not feasible to completely remove).

This would then give people the freedom to choose how they wish to proceed with level design in full light of community guidance, and the same things don't need to keep being repeated. Newcomers can simply be referred to the Community Level Design Guidelines.

I'm not going to take a position at this time on whether or not the warning should exist, but with the proposed use cases, I'd like to say that I think there are legitimate use cases for multiple overlapping windows - for instance, creating a level where every other spawned lemming has different permanent skills. There's certainly some deceptive ways to use it, of course, like overlapping a bunch of hatches to make a bit of a "gotcha" lemming to break a solution that otherwise should have worked.

This is at least partially noticable because of the weird-looking mess created by the overlapping skill icons. Even this isn't ideal. I think I have a couple of levels that did this back when fairness wasn't as strong a concept as it is now; I should change them. Or, improve how this is displayed, then it could be accepted as legitimate.

I think it's important for the helper system not to get too messy: maybe some things could be made visible from the panel, or when hovering the cursor over an entrance hatch, for example (for the latter, maybe hovering over the hatch could display all info about that hatch as a list: spawn order, number of lems, permanent skills if applicable, whether it's overlapped...) and in the event of overlapped hatches, simply clicking on the hatch could display the info about the hatch underneath it...

Mayyyyybe too many things to have to know about, but I suspect that most players will simply just play the level and find out the information they need to know by simple observation. The feature would mainly be there for those who prefer to know everything about a level before they start playing it, and such players are likely to familiarise themselves with all of the interface/helper features anyway.

What about exits or other objects partly covered by terrain (as to simply require an additional skill to get access to them, or to allow a worker lemming to walk past it earlier, similarly to a locked exit)?

What about exits set to no-overwrite and placed entirely on top of terrain?

I think it would depend very much on the coding whether these would be caught as false positives.