While I don't know of any such site, I wouldn't recommend it either.
Open source projects are made by the community, for the community, so I'd recommend you start by helping out one of the projects that you personally use. Maybe you can fix a bug that is bothering you, or implement a functionality that you are missing.
Can I ask why you wouldn't recommend it? I'm open to the possibility that I'm missing something but imo exposure and attention would often be a big hurdle when it comes to open source projects.
Having a repository that lists and describes them doesn't feel like it could be a bad thing to me
I believe what they might mean is if you don't personally use a project, you might not be as committed/passionate about it as someone who uses the software in their daily life and cares about it as a user.
So it's not about finding one with lots of users and dropping in there if it is software you don't care about.
If you are just getting into using open source software though and have yet to find any that are important to you, then you can usually find lists of open source alternatives (from a user perspective) with a search fairly easily to get to better know what is out there and what you might be most interested in. (And lists like that for people to find software like that are generally helpful, I found lots of tools I wouldn't have otherwise known about through peoples collections of favourites/alternatives/etc.!)
I have looked for something similar. There are a number of spaces where FOSS project lists are maintained, but they are often focused on a singular topics like 'privacy' or something akin, and they aren't often parts of larger lists that can be sorted based on the conditions you mentioned above.
The closest thing, if you are interested in other possible tools that might help: Alternative.to, a crowdsourced software searching tool, which has a means of filtering to show only, say, open source projects, or sort by tags that denote stacks used, languages used, etc. (see screenshot of tags I added). It has been useful enough for my own needs when looking for what you've been looking for.
Either way, best of luck! I haven't been able to find something yet, myself.
If you're an Android user: scroll through F-Droid, download interesting apps, and then hunt for problems or try to come up with interesting, helpful features. For example, I could really use a live stopwatch widget in: https://f-droid.org/packages/com.best.deskclock/
Google Clock has it, but this app doesn't for some reason.
Gitlab will be down for a week (or more) due to server migration operates under a freemium model. GitHub is having weird Microsoft money making issues. Codeberg is the "non corporate" alternative and I see it being mentioned more and more.