I see a number of threads over the years that have requested global tagging across accounts. But AFIK it has never been implemented.

I think this is a major missing feature. Anyone know if there is any chance of it happening?

Another feature which would be nice is being able to exclude accounts from a search. Or at least having a column that shows which account a message returned in a search belongs to that can be sorted or and/or filtered on. When you do a search, and it returns a bunch of messages, AFIK there is no easy way to even see which account the message belongs to without opening messages (in some cases the "To" address cannot tell you which account it was sent to).