One issue is gems which are locked `ruby < 4.0` which will now require updating, and releasing 4.0 instead of 3.5 was only done very recently.
For a more concrete example, the grpc gem locks Ruby versions (< 3.5), and they refuse to change it. So until they support the next Ruby version, we could test ruby-next by testing with a preview release. This worked for 3.4 and 3.5, but now doesn't work with 4.0 (bundler resolves 4.0-preview2 > 3.5, whereas we are able to do 3.5-preview1).
So unless I feel like doing a lot of grunt work (which I don't), I can't even test Ruby 4 in our app until they release a new version. And while I recognize this is an issue with the gem, it is a consequence of choosing to do 4.0.
For a more concrete example, the grpc gem locks Ruby versions (< 3.5), and they refuse to change it. So until they support the next Ruby version, we could test ruby-next by testing with a preview release. This worked for 3.4 and 3.5, but now doesn't work with 4.0 (bundler resolves 4.0-preview2 > 3.5, whereas we are able to do 3.5-preview1).
So unless I feel like doing a lot of grunt work (which I don't), I can't even test Ruby 4 in our app until they release a new version. And while I recognize this is an issue with the gem, it is a consequence of choosing to do 4.0.