“It’s not a bug, it’s a feature” …
This was a bit hidden, and frankly I did not expect such a feature (as default).
It’s actually quite important for scaling. Calendars may have thousands or more events in the past (lets assume a calendar with 2 – 3 events/day * 365 days = 1000 events/day * 10 years = 10000 events), which shouldn’t be checked at every sync nor downloaded at all because in most cases, people won’t need it and it causes much traffic and potential problems.
Also it works not quite the way the setting in the app describes it: (translated by me from German)
Appointments older than 90 days in the past will be ignored
To ignore something means for me, not to do something with it. But DAVx⁵ does indeed virtually delete the local entries. This was not expected if I interpret the option as described
and was kind of misleading.
I agree that this wording is not exact. DAVx⁵ has (like CalDAV in general) a server-centric view, so it means that events older than 90 days on the server will be ignored and not downloaded.