Parser cache hit ratio alerting is difficult, specially on a passive DC. A better option would be a script that checks that most of the content is not expired, aka "does not contain mostly garbage". That together with "replication is running" would confirm the parser cache has a good status without actually checking the hit ratio. A 2/3 relation of usefull content globally would be enough to allow for a single server to fail without the alert going off.

My suggestion for this kind of check was not for the passive dc, but mainly the active one to make sure that the parser caches are properly used. We might have changes in mediawiki that will change the hit ratio over time and it could go below a threshold that causes issues.
I think it might be useful in general to have this check and it would have also immediately alarm after the switch to tell us the real cause of the issue. It's not meant to prevent it, for that we'll have the other ones (replication/heartbeat/cookbook)

I don't think having such alarm is bad- it is easy to setup, just setting up a prometheus one- but it may arrive too late. I think a check on switchdc would prevent issues rather than identify them afterwards- and it should be trivial to add pc1 pc2 pc3 to the list of checks to test, let me see the code.

That's exactly what I meant, we should have this check independently and adding other checks to the other part described in T207385 to prevent it.