Paul,

Groups.io digests strip out the message that is replied to, so in our
test group this led to nearly unintelligible digests. Is it possible
to change groups.io digests so they mirror Yahoogroups behaviour?

I've wished for limited quoting in digests. By limited I mean just the direct quote (first-level) and not the quotes that quote drags in. One of the defects of the Y!Group digest is that it can get unreadable when a message quotes a message and that message quoted prior messages, and so on.
https://beta.groups.io/g/main/message/1433

Later I proposed a snippet of the quoted message, rather than the whole quote, might provide sufficient context in the majority of cases. That is probably simpler to implement than my prior suggestion (so more likely to be implemented). It would also have the benefit of being briefer, keeping the Digest more readable.
https://beta.groups.io/g/main/message/2267

PS I’ve found a way to fudge the issue (by turning on the signature
block) for all users except those who reply via the web interface
without quoting the message they are replying to.

Hm... That might represent a bug of sorts. You mean the quote detection fails when there's a trailing sig applied by the group?

“Educating” our users is unlikely to produce the required result.

That is the bane of all email systems. Well, maybe all systems.

Shal