We didn't mark it as a feature of GitLab.com since the company decides if it is used, we also pay for the hosting (memory) cost, and there should be no end-user difference between Puma and Unicorn. But I agree that there is an argument for marking it as available there since all of GitLab.com runs on it.
Puma reduces the memory footprint of GitLab by about 40% compared to Unicorn.
Considering one of the most frequent complains about Gitlab is memory usage. 40% is huge.
[1] https://about.gitlab.com/releases/2020/05/22/gitlab-13-0-rel...