One example I can think of was the Play framework.

If you wanted to work with Scala 3 and Play 2.7, you couldn't. You needed to stick with Scala 2.13 to be able to use Play 2.7.

This was more than a year ago. Now we have Play 2.8. Is it compatible with Scala 3 now?

Edit:

I am not entirely sure whether it was possible to make Scala 3 and Play 2.7 work. So my statement above might be partially wrong.

However, I had other dependencies too, which required me to use Scala 2.13.

Consider to downvote this, if you think I made a false statement. I should have looked up the documentation beforehand and not rely on my (past) personal experience/impression at all.

Bad mistake!