Outlook has this and I've been using it in a work context for many years. Create custom tags for domains and or projects, tag email, notes, meetings with as few

Outlook has this and I've been using it in a work context for many years. Create custom tags for domains and or projects, tag email, notes, meetings with as few or as many tags as you like, and create handy 'Search folders' which are basically a view on a query (you can use other query terms beyond tag of course). The actual email just goes into big ol' "Archive". I gave up on folder heirarchy many years ago precisely because some emails map to multiple issues/domains/projects.