I can confirm that setting `nnimap-split-download-body-default' to t
works as intended.  I really should have tried that first.

It sounds like the removal wasn't intentional, so hopefully it can be
added back.

It seems to download the body before the splitting, which is
unfortunate.  It would be better if it only downloaded the body when the
split required it.  Or if it could match the sender against a whitelist
of accepted senders and only download when they matched.  Or the same
thing but with the summary, but then it's getting close to just adding
a simpler split system before the real one, which is a little odd.

So I don't forget, someone also suggested `nnimap-split-download-body
size', which might be a useful addition: