The 3.x branch is the only branch that's compatible with groups 3, however it also introduced a plugin ID change without an upgrade path, see ✨ Change GroupRelationship id for compatibility with subgroup module Needs work for more background.
To allow sites to update to group 3, I think the following should happen:
1. Open a new 4.x branch with the current code from the 3.x branch, include a release note that there is currently no upgrade path from ggroup 2 or lower versions to it.
2. Revert the plugin ID change in the 3.x branch, and release a new release candidate with a release note that any existing 3.x users should upgrade to 4.x immediately.
3. In [ #3209627], add the upgrade path to 4.x
Opening this issue for part #2.
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