Solution-optimisation is slightly different from speedrunning in that the point of it is to perform an intended/fair solution in the most efficient way, rather than find a way to bypass it or make use of glitches/computer-assistance/backroutes to find the fastest possible way to complete a level.
To be clear though, a time limit wouldn't do anything though when it comes to people bypassing a solution via glitches or backroutes, you'd still have to fix the backroute or the glitch to make it not happen. And not sure what you mean by "computer assistance". If you are referring to use of framestepping and similar tools, how do you know someone can't execute a solution without using those features given enough practice? In a lot of games, tool-assisted speedruns actually tend to help with human-executed speedruns as well, with the tool assistance conclusively proving that something is really possible, and then the human goes about practicing the execution required.
Also, in many other games, speedrunning generally have multiple variants to account for things like glitches. For example, a game that has a major glitch allowing one to skip almost from start of first level right to the ending, there will of course be a speedrun category where there are no rules and allow for such shortcuts as record solutions. But there'll probably one or more other categories/variants that disallow the use of that particular glitch, or one that specifies all the levels must be played through.
I think the biggest disadvantage Lemmings-style games have when it comes to speedrunning, is that the game just isn't very speedy like your typical action/platformer games. Speedrunning-level execution on those types of games generally require very good reflexes and quick fingers, and this is where the excitement primarily comes from for both the player and the spectators. Lemmings on the other hand, typically wouldn't really involve such things even when you are executing an optimized solution. If it's really just about finding the earliest time to release the crowd for example, once you worked that out for a level, there isn't much excitement left to execute or watch someone execute that timing. This is why in Lemmings, speedrunning only becomes more interesting for most people when it also involves actually doing something more than what you typically do in solving the level, so that the time record solution actually features some novel actions (which often don't have to be glitches or backroutes), rather than merely the exact same actions as your typical solution just done at the correct times.
Things like these should not be discouraged at all .
To be clear, I don't recall there ever been a public discussion here about time limit talismans. WillLem is the first person who brought it up, seemingly in the context of someone (in the singular probably, I emphasize) privately giving some criticism about his inclusion of such in a level. I don't see that as being discouraged, unless you insist that every single criticism, even when only been voiced once by one person privately, is "discouragement". As I said, you can't please everyone all the time, this is a fact of life even outside of this forum. It is accurate to say that the popularity and intensity of objections to time-limit talismans had been far, far lower/quieter compared to something like use of hidden traps in levels, even before WillLem started this thread. (If you disagree, please point me to some past posts and threads that even touched on time-limit talismans.)