Lemmy

I personally wouldn't be opposed switching off github once forgejo or another fully federated issue tracking system becomes mature, but the rest of the main contributors would have to agree also. Until that time, you'll have to either use github, or get someone to make a PR for you.

Short answer: no

Long answer: no, but longer

Just like you can't directly contribute to Gnome without using gitlab.gnome.org, you can't directly contribute to Lemmy without GitHub.

Or rather, easily. If you're submitting something simple, you could communicate with Lemmy Devs over Matrix and ask if they're amenable to receiving a git patch they can create the PR for. https://join-lemmy.org/docs/contributors/01-overview.html

I remember looking at the code hosted somewhere else once or twice, and then again recently and it was on Github now and I was a bit surprised. Wondering what happened there.

You should be able to mirror or fork, make contributions there, and you can probably try to do a PR back into the original. You will still have to use github somewhat since that's where the original repo is.

You can't. Even if there are mirrors elsewhere, you have to use github at least indirectly to contribute to the source

Given the rise of anti-US sentiment and calls to stop using their products, more people leaving Github might be a real occurrence

Highly unlikely. Github has nothing to do with current politics and has massive network effect. There is no way in heaven or hell you'll see much effect on Github from this