Sorry to hear that. Are you sure that the render pipeline and project settings have not changed in regards to e.g. shadow casting on secondary lights and similar settings (or perhaps new settings were added)? You could additionally perform a diff on the whole ProjectSettings
directory and additionally on the render pipeline assets of your 2019 and 2021 projects.
Another thing that comes to my mind is that perhaps Unity's batching behaviour has changed, from aggressive batching to (more correct) conservative batching. Is the framerate also worse when only using a single Spine skeleton in the scene, and nothing else? Also, is there any framerate difference when removing all Spine skeletons from the test scene, and comparing this same scene in your Unity 2019 and 2021 projects?
Regarding the thread topic: unfortunately that can't be changed, you could only delete your post (and thus implicitly the thread) and start a new thread with the correct title. No worries however about the wrong version number in it.