What version? I don’t know!
Use the last released version, 9.0.1
There is no hotfixes for previous version of VTK.
There seems to be a lot of API changes in the pipeline, how are others keeping their codebase stable?
Every single API changes implies a legacy code to ensure retrocompatibility. This is part of the VTK code guidelines.
There are so many merge requests, must be a lot of thrash on the master?
Each merged Merge Request is reviewed and merged by experienced members of the VTK community.
And what is the “release” branch for exactly? Is this a “more stable” version of master?
The release branch is used for new patch release (eg: 9.0.2 if it ever comes) and only contains bugfixes, it does not contains new features as master does.
I’m wondering if I should just be branching of version 8.20
You definitely should not. the VTK 9 breaking changes are for the better.
I should also mention that this would be a good page to document the release plans: hasn’t been updated for 9.x at all yet …
Indeed, the wiki is being phased out. You can find documentation about the VTK 9 changes and release notes here: VTK 9.0.0 - #6 by ben.boeckel
That being said, we definitely needs to improve and clarify this process.
Are you a VTK contributor @Paul_Harris ? Maintaining and improving a opensource project as complex a VTK is a very big task and we will take any help we can get.