why is 15.11.13 a required upgrade stop?

I see it in https://docs.gitlab.com/ee/update/#upgrade-paths and https://gitlab-com.gitlab.io/support/toolbox/upgrade-path/?current=15.7.7 but I'm not seeing anything on https://docs.gitlab.com/ee/update/versions/gitlab_15_changes.html or https://docs.gitlab.com/ee/update/versions/gitlab_16_changes.html regarding why it's required.

cc @bprescott_