Spotted this Hacker News discussion:
https://news.ycombinator.com/item?id=20342060
of Gitlab's blog post "Why we're ending support for MySQL in 12.1"
https://about.gitlab.com/2019/06/27/removing-mysql-support/
and what features of Postgesql that MySQL lacks made them decide to drop MySQL support.
In all honesty, that they didn't use MySQL themselves and had low MySQL usage, was probably the most decisive. Still, discussions on relative advantages of Postgresql amount to advocacy in my book.
Cheers,
Jeroen