Scala 3.4.3-RC1 hotfix release is now available for testing: Release 3.4.3-RC1 · scala/scala3 · GitHub
This release reverts changes introduced in Scala 3.4.2 that led to binary incompatibility of match types produced by the compiler. You can read about this issue at Auto-approximating bound for match types caused changes in erasure, which are TASTy- and binary incompatible · Issue #21258 · scala/scala3 · GitHub. The new behaviour would be restored in some next major version (probably 3.6) after being well described and probably introducing a warning for the users.
We recommend authors of Scala 3 libraries that use match types in their APIs to set up TASTy MiMA checks as the extension to classic bytecode-based MiMa binary compatibility assertions.