Dieter Maurer notes that loadBefore cannot be deprecated yet because ZEO
essentially depends on the `end_tid` information returned by loadBefore
to update its cache:

https://github.com/zopefoundation/ZODB/pull/323#issuecomment-842021970

And to remove this dependency it would require to rework ZODB caching layer:

https://github.com/zopefoundation/ZODB/pull/323#issuecomment-845917355

So we cannot deprecate loadBefore until this rework is implemented first.

-> Remove general loadBefore deprecation, and emit loadBefore vs
loadBeforeEx warning only when actually hitting a "deletion" record,
because only that case is known to lead to data corruption.