You are misinformed; they implemented a Sentry-compatible endpoint if the Project enabled it: https://gitlab.com/gitlab-org/gitlab/-/blob/v15.0.0-ee/lib/a...

I feel perhaps you meant to link to https://gitlab.com/gitlab-org/gitlab/-/issues/329596 which describes their intention to accept Sentry formatted JSON payloads and store them in their own database. Searching for "no need" in that issue will highlight the number of times they were drawing a distinction between the effort required to run GitLab and the effort to run the 15 containers of Sentry self-hosted