Hierarchical grouping was an experiment which never came to fruition, and we have since removed any code using its results. It can therefore be removed, but we have to do so carefully, because we still call the code to generate hierarchical hashes in a number of places which are still live code. The most notable among these is in the mobile grouping config, which a handful of projects are still using.

Code we can remove now (actually dead, probably not a full list):

• GroupHashesSplitEndpoint, its associated URL, and test_group_hashes_split.py (the last references to the endpoint were removed in mid-2021 in ref(merged-issues): Remove frontend grouping related code #26733) - Done in chore(grouping): Remove unused GroupHashesSplitEndpoint #72552
• GroupHash.state enum option SPLIT (there are no GroupHash entries which have this state; do we need to run a migration to switch them all to None anyway, to handle on-prem folks?)

Code we might be able to remove now (needs investigation):

• Code dealing with hierarchical tree labels (hierarchical_tree_labels appears to be written but never read, but the other values set in _write_tree_labels still need to be investigated. Also look into current_tree_label, finest_tree_label, and display_title_with_tree_label.)

Code we can't remove until no one uses the mobile grouping config:

• TBA

Code we probably can't remove until no one uses the mobile grouping config AND there are no events with hierarchical hashes:

• The HierarchicalGroupingContent component

Other tasks:

• Figure out how these changes affect (or don't) on-prem users. Do we need to deprecate the mobile grouping strategy before we delete it and the associated hierarchical hashing code?
• Force people still using mobile to upgrade their grouping config, regardless of their auto-update setting. We'll actually need to do this twice, so that the mobile config isn't their secondary config, either.