From T303828, I have mentioned using branch.py --core --core-bundle wmf_core --bundle wmf_branch --delete <VERSION> which has support for converting branches to tag (via T250095 and c3f8dc8e96b8c91f11444cc53f6b2b7dcc7f702c).
I went to run it for release branches with a dry run (--noop) mode:
$ ./branch.py --noop --delete --abandon --bundle wmf_core REL1_19 Repo mediawiki/extensions/3D doesn't have a branch named REL1_19 Would create tag REL1_19 pointing to c103fac20fa7dd63d439ce2817901d89f6d368ec Would delete branch REL1_19 in mediawiki/extensions/AbuseFilter
But when running it without --noop:
$ ./branch.py --delete --abandon --bundle wmf_core REL1_19 Repo mediawiki/extensions/3D doesn't have a branch named REL1_19 Created https://gerrit.wikimedia.org/g/mediawiki/extensions/AbuseFilter/+/refs/tags/REL1_19
It has deleted the branch and created the tag pointing to master!!!