Do they have backwards compatibility guarantees?

Otherwise what are we doing here?

They do, some of the best of any language.

That said backwards compatibility problems still hit as some libraries enjoy using internal APIs.

It's not an every time thing and it's been easier and easier with updates.

I work with dotnet but my understanding is that some applications/ teams are still on java 8 with spring boot or whatever so it isn't like they aren't modernizing but they are choosing to do so at their own time which is fine I think