It's been a while since last ZODB3 3.10.7 release in 2016 and the last
commit in upstream ZODB3 repository (3.10 branch) is from 2017. The
world switched since then to ZODB4 and to ZODB5 after that.

We were still requiring ZODB3, because ZODB3 3.11 egg was just a
dependency on newer ZODB, ZEO, BTrees and persistent; and this way we
could be supporting all ZODB3.10.x and  ZODB4 and ZODB5 via ZODB3.11.

However upcoming Wendelin.core 2, for its proper working, needs MVCC
semantic as implemented in ZODB5. This forces us, even for ZODB4, to
backport non-trivial bits from ZODB5 (see [1]). Maintaining ZODB3
support at this point becomes non-practical, because, to our knowledge,
there is no wendelin.core user that plans to continue using ZODB3
without switching to at least ZODB4 in the near future.

So goodbye ZODB3. Even though ZODB still stays with us, it gives a
feeling similar to [2], because in 2014, when I was myself learning
ZODB, it was through ZODB3 - still at the time when all ZODB bits were
living together in one place.

[1] ZODB!1
[2] https://lists.osuosl.org/pipermail/darcs-users/2008-September/014095.html