I can confirm I have multiple inflight CMS 12 projects currently using Geta.NotFoundHandler.Optimizely @ v3.0.1 and I'm not observing any issues at the moment. However the project was renamed from Geta.404Handler to Geta.NotFoundHandler when building for CMS 12 so as part of upgrading from CMS 11 to CMS 12 you will need to uninstall Geta.404Handler and install Geta.NotFoundHandler.
There are a hand full of other CMS 12 targetting packages made by Geta where they have been renamed for CMS 12 and for the rebranding from EPiServer to Optimizely, for example Geta.EPiCategories is now Geta.Optimizely.Categories.
We are upgrading our CMS version to 12 and realized the old custom redirect manager doesn't work in CMS 12.
Looks like GETA doesn't work with CMS 12 either.