You’re on the open-source community, of course we’ll be biased in favour of open source. One thing to point out is that open-source and closed source are both pretty broad categories that cover several licenses. Source available means people can see the code, but there are restrictions to how they can use it. Is there a specific thing you don’t want people to do with your code? Do you not want them to edit it for example? Or you’re fine with them editing it, but not for commercial purpose ? Any restriction of this type will make it source-available. If you’re fine with them doing anything, it’s open source. If you want them to mention somewhere that their code is based on yours, it’s still open source. And if you want any code made by editing yours to also be open source, that’s still open source (that’s the idea of the GPL). But other restrictions might make it not fit that category.

I personally usually default to the GPL3, I’m fine with people doing anything with my code except making it non-open source. Well “my code”… It might be a bit presumptious of me, I’m not really a programmer, I’ve just made a few small and not very useful things. There may be legitimate reasons for not wanting your code to be open source sometimes, but for me the stakes have always been low.

As for whether using Github creates an expectation for Open-Source… Not so much at this point. It’s very used by the Open-Source community, but not only. Plus, it’s not really open-source itself, so the most purist prefer other git platforms like git-lab, forgejo or source-hut.