What does a version being in "supoort phase" via https://umbraco.com/products/knowledge-center/long-term-support-and-end-of-life/ mean for contributions? Would I waste my time if I suggest a change to v13 and then create a pr for it?

We still accept bugfixes for v13. Support phase focuses on making sure the version is solid and stable, bug fixing fits in very well there 🙂

What I had in mind would be more alongside a feature request, but I think I'll try to create an issue as it works the same in v14 so even if it wouldn't fix my immediate problem it would be good to have the conversation anyways 🙂

In general, we're not putting new features in, so probably not going to make it.. but maybe if it's not so much a feature but more of an improvement.