From my limited perspective I agree 8.90 should be a candidate for a major version bump.
It will almost certainly be 9.0 upon release. But, we don’t want to call it 9.x yet because otherwise in-development code will be detected as 9.0.
Anyhow, CMake treats the 8.90 branch like any other 8.x branch
This is because of this change which I objected to when proposed for ParaView and the rationale described there applies to VTK as well. Please file an issue referencing the VTK merge request and Cc the MR author there for discussion.
But for posterity, I was surprised because even though 8.90 is not released, it’s behaving as if it is a released variant of VTK 8.x according to CMake.
There’s no good way to indicate “is released” in a version number without pre-release versioning which isn’t really standard. The closest we could get is calling it 9.00.0 which is less than 9.0.0 in CMake, but is “only” extremely confusing to everyone else.