retention comes with insufficient warnings

[richvdh]

it is well-known to corrupt the database via insufficient cache invalidation, yet the config documentation almost recommends it. It needs a big health warning

[richvdh]

cf #13476

[4nd3r]

What happens if I decide (because of mentioned risk) to remove retention: enabled: true from configuration and many rooms in my HS have retention configured? Is it safe? Please add that to docs also.

[dklimpel]

And are there different risks if I delete history e.g. from yesterday or 3 years ago?

[richvdh]

What happens if I decide (because of mentioned risk) to remove retention: enabled: true from configuration and many rooms in my HS have retention configured? Is it safe?

I believe that's fine, yes.

Please add that to docs also.

Pull requests to improve the documentation are always welcome 😇

[richvdh]

And are there different risks if I delete history e.g. from yesterday or 3 years ago?

Yes, there are different risks in the sense that you're less likely to hit one of the problematic races by deleting history from 3 years ago than by deleting history from yesterday.