@d-maurer suggests[1]:

    The ZODB logic relating to historical data (including MVCC) was largely
    centered around before. You have changed this to at - requiring wide spread
    modifications. I would much prefer to keep the before centered approach...

    (https://github.com/zopefoundation/ZODB/pull/323#pullrequestreview-650963363)

So let's change "at"-based logic to "before"-based logic and rename the new
method from loadAt to loadBeforeEx.