Whether the skill remains in the stable version or not, I hope the experimental version remains available in case it is removed from "main NeoLemmix" again. Just like 1.43 remained available to play gimmick- and 10.13 remained available to play radiation/slowfreeze/anti-splat pad levels.
The difference is that 1.43-F and 10.13.18 were stable versions, not experimentals. And for those, creation of
new content for them is strongly discouraged, as you know - they're kept available so that older content that hasn't been (or can't be) converted to newer versions can still be played. No promises are made regarding the ongoing availability of experimentals and you absolutely should not
ever release content that relies on an experimental feature that hasn't yet / never did (as applicable) make it into stable versions, except maybe if that feature is purely cosmetic. (I don't mean this in the sense of "We won't let you do it, we'll delete your posts, etc" - we're not going to do that - but simply in the sense of "That's a really, really, really bad idea.")
Adding the Shimmier is Nepster's thing, so it's really up to him to make the call on whether it stays or goes. I don't personally see any reason not to keep it. EDIT: And Nepster has stated the same, that Shimmier is almost certainly going to be included in the next stable release, with few (if any) further changes. I would still advise not relying on any very precise details of its mechanics (eg. don't create a ceiling, if you can avoid it, that's
exactly the height a reacher can reach - give a few pixels of wiggle room. Likewise, don't push it to the limit with slopes, etc) just yet, but it's probably safe to use it in general - just keeping in mind you might need to tweak your levels for a version or two of NL while any potential remaining wrinkles are ironed out.