It can be a name that we keep for longer. I think we should still warn that it is deprecated. Note that 9.5 is the earliest that we will remove 9.3-deprecated code; there’s nothing saying we cannot provide it for longer for more widely-used things.
The idea is that code supporting 9.1 and 9.3 at the same time can use the same code. VTK-using code can use #define VTK_DEPRECATION_LEVEL VTK_VERSION_CHECK(9, 1, 0) to say “I should work with 9.1, please do not warn about names which were deprecated afterwards”. Note that 9.3 will ignore this as 9.1 symbols are on their way out the door and force the warning since removal is much more urgent at that point.
As for the MR itself, I noticed that some methods seemed to have been removed in the process here in various places; is there any way we can keep those around as deprecated? If they’re not accessible from outside of VTK for whatever reason, then it’s fine.