So I go and change them, but then people responding via e-mail end upIf Mark were looking for something to do there's a solution to this problem.
When splitting, merging, or renaming a topic - any time the Subject text can be changed - Groups.io could retain a copy of the prior Subject text for all the affected messages. Or I should say a list of prior texts, as a moderator could make more than one edit (or split, or merge) affecting any given message.
Then when an inbound message is being evaluated for where it threads those prior texts would be available as potential matches to the inbound subject line.
In cases where the inbound message includes an In-Reply-To and/or References field this restores the unambiguous match to the original message, sparing the moderator of the necessity of doing it.
In most cases of Edits and Merges there would likely be only one topic (within the two-week cutoff) with a matching Subject text, either present or past.
Splits would leave one topic with the original subject (as now) and one topic with a new subject plus the original in its history. Absent one of the referencing header fields an arbitrary choice must be made as to which of those two topics the inbound message belongs with. Choosing the topic with the most recent message is probably the least wrong choice.
File this as a "not quite FUSSP"
(Final Ultimate Solution to the Split Problem)