Requiring time limits
This may differ depending on your opinion, as quite a lot of challenges have time restrictions, but, for me, this is a form of requiring a minimum score.
If a challenge is code-golf, unless the specific goal of the challenge is to do the task quickly, adding a time limit simply prevents people from posting good answers and adds nothing positive to the challenge. Older challenges are more guilty of this, but occasionally a new challenge will be posted requiring answers to finish within X minutes.
A lot of challenges that contain a time limit often do so in an attempt to make the challenge more interesting. Banning specific builtins, classes of languages or other arbitrary things has been repeatedly shown to not make a challenge more interesting (see the other answers in this thread), so requiring answers to meet a specific time limit is unlikely to as well.
Furthermore, timing is non-perfect. What takes 5 seconds on one computer may take 50 on another. That's why we require specifications to be provided for fastest-code, otherwise it's no longer objective. TryItOnline has a time limit of one minute, so could possibly be used for such a time restriction. However Dennis - the host of TIO - has said repeatedly that TIO isn't reliable enough for fastest-code, and so probably isn't accurate enough for time limits.
The worst culprits in my experience are chameleon challenges, which detail an inefficient algorithm/method to complete the task, then all but outlaw that method by requiring answers to meet a time limit (see a lot of the older combinatorics challenges for example)
Obviously, this is a matter of opinion; some people prefer to see solutions finish, others don't mind. However, when considering adding in a time limit, ask yourself if it actually improves the challenge or just prevents otherwise valid answers from being posted.
As an aside, this isn’t to say that you should never include a time limit. Challenges where the sole aim is to “Do this thing quickly and golfy” are the main areas where a time limit works. Challenges where the aim is “Do this thing golfy. Oh and make sure you choose an algorithm that isn’t too slow” are a poor idea. Instead, look into either fastest-code, or consider using an asymptotic complexity limit, restricted-complexity, which is much more objective.