Okay I've clarified the situation. Hiding / unsubscribing folders does yes work for custom folders, but it does not work for 'standard' folders: Drafts, Sent, Spam/Junk, and Trash. These folders have checkboxes in the Subscribe dialog, but un-checking them off has no effect. 'Archives' behaves as a normal/custom folder, not as a standard/permanent folder; it can be un/re-subscribed at will. But because it can be automatically created, it sits somewhere *between* normal and standard.
On some mail accounts the standard/special folders are underneath the Inbox, and on some accounts they are *beside* the Inbox. Either way, they
tend to be always visible in the account-tree pane. This is bad because I never need to see them on most of my fifty accounts across five mail providers. The way this works is - on one provider for example, maybe five accounts are incoming, and one account is outgoing. All incoming accounts never send mail under their own address/SMTP login. Instead, they all log in to the one outgoing account for sending. Further, they all store their templates/Drafts/Sent mail in the outgoing account. Thus, only the one outgoing account needs to display the Drafts+Sent+Templates folders. On all the incoming accounts, those entries in the tree pane are only wasted vertical space, because those folders never contain anything.
The folder subscriptions feature has various other annoyance-bugs too. And, this workflow exposes another bug, named "Draft Autosave fails before the outgoing account logs in".
Anyway, this is a request to allow hiding whatever folders I feel like, maybe even through a preference that enables unsubscribing from the default folders.