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I started building Multiverse because of problems like this. Ironically it’s hosted on GitHub. Check it out if you are interested in decentralized VCS and code hosting.
GitHub uses MySQL, not Postgres. They built the best-in-class online schema change tool gh-ost [1], and have a custom declarative schema change execution system built around Skeema [2], which contains a wealth of linters [3].
Even so, it's always possible for an engineer to submit a schema change which is detrimental to performance. For example, dropping an important index, or changing it such that some necessary column is no longer present. Linters simply cannot catch some classes of these problems. Usually they must be caught in code review, but people make mistakes and could approve a bad change.
Disclosure: I'm the author of Skeema, but have not worked for or with GitHub in any capacity.
[1] https://github.com/github/gh-ost
[2] https://github.blog/2020-02-14-automating-mysql-schema-migra...