I mean, depends on your MUA in practice. I think by standard for server side threading— https://tools.ietf.org/html/rfc5256 — "base subject" is definitely a valid consideration. I only skimmed though. :-) feel free to extract a spec, I'm curious (but not enough to read 19 pages right now)
ORDEREDSUBJECT
The ORDEREDSUBJECT threading algorithm is also referred to as
"poor man's threading". The searched messages are sorted by
In other words: as a workaround for garbage email clients and services too incompetent to use email correctly.
Yes, the REFERENCES algorithm also uses the base subject--but never to override message-id based threading, only for merging together threads that couldn't be joined using message-ids ... or in other words: as a workaround for garbage email clients and services too incompetent to use email correctly.
Well, and for merging together chunks of a thread where you have major gaps in your mailbox, so that reference headers don't overlap enough to make the connection, but the point remains that it's not really a threading mechanism, just a last-ditch attempt to recover a thread where proper threading information is missing, which is fine for corner cases like large gaps in which messages you have in your mailbox, and doesn't change that a sender that doesn't supply reference headers in the first place is incompetent.
