POP3 "delete" did fully purge, never left any "remnants" in trash.
I don't like IMAP, because it is somewhat of a "mirroring" service, but doesn't offer what I want.
I want servers to fully purge after download, and the way Yahoo and Gmail do IMAP, they only move to Trash but do not purge, and no option provided to do that, forcing manual login to flush the trash. 
Actually, e-mail clients may differ. IMO, Thunderbird can imitate POP3 behaviour while using IMAP.
Account settings, under Server settings it is possible to select When I delete a message > Remove it immediatelyTools > Message filters menu it is possible to create a filter which would match all new messages and move them into a local folder.Thank you, Eugene, but I am already doing what you say, and both, Gmail and Yahoo (via my ISP), are interracting via IMAP ... BUT ... the remove immediately does not remove immediately. It only moves to the server trashcan!
That an interesting twist of the plot, Eric. I'm ready to blame mail servers for not respecting 'Remove immediately' option... But before that I'd suggest to turn on synchronization on Trash folder to possibly modify that behaviour. It turns out that sync is not switched on by default on all folders. (At least on my mail client).
When you say "phasing out POP3 support", do you mean the ISP is blocking that port? Or are you using ISP provided mail servers (e.g. AOL) that are dropping POP3 support? Haven't heard of POP3 being phased out on web mail providers (e.g. Gmail, Yahoo).
Wouldn't surprise me if they block POP3 for incoming requests to a residential POP3 server: misconfigured mail server = bad IP address = bad rep.
Never really used POP3 myself, but I liked the idea of it being a digital postbox/mailbox 
that you would 'pop' open to grab the letters 
and take them home 
Well, it seems that as GMail as Yahoo mail do support POP3 protocol over SSL.
At the same time I would not be surprised to find out that some mail providers might not support POP3 (and, probably, IMAP). IMHO, nowadays web-based mail clients slowly send standalone mail clients into oblivion.
With IMAP, the whole concept is synchronization/mirroring.
With POP3, the concept is client independant of server, not a mirror, but a only a delivery point, with the server acting only as transient holding basket until it can be delivered.
They are never intended to be "mirrors" of one-another. It's like the server having cubbyholes for various items going in and out.
The POP3 server's inbox, containing items, would be grabbed by the "Postman", and handed over to the inbox of my desktop client. Upon confirmation of successful delivery in desktop Inbox, the "Postman" is supposed to delete immediately, not move to the Trash, which is what the ISP servers are now doing (both Gmail and Yahoo, with their supposed POP3 service), which is because the ISP no longer has a proper POP3 but is emulating it using an IMAP back-end. Otherwise, the delivered mail would be immediately deleted, as per proper POP3 protocols and servicing.
Same problem exists with the sending of email. If I sent email, thru POP3, I expect the "sent" mail (as per past behaviour, which changed about around 6-8 years ago) to be deleted immediately from the server, not preserved in a "Sent" folder, which is again what both Gmail and Yahoo do, which again is IMAP behaviour, not POP3 behaviour. I think that happened about the same time that both providers deployed the OAuth capabilities, which for which POP3 may not have been adaptable.
I choose to stay with POP3, because of its defined/intended interraction, but both Gmail and Yahoo have "corrupted" their server code to behave differently, my suspicion being that they want to their normal backups capture the passthru image of every item transferred via email, for their own, or regulatory reasons!
Are they using something like this, instead of running 2 different mail servers (namely only IMAP)?
To support my claim, here is an excerpt from the Dovecot manual relating to POP3 handling by its IMAP server:
Migration from Gmail to Dovecot
You can use dsync migration via IMAP protocol, but there are a few things different with Gmail compared to other IMAP servers.
With Gmail, when you delete a mail from POP3, the mail is only hidden from future POP3 sessions, but it's still available via IMAP. If you wish to preserve this functionality, there's a pop3_deleted_flag setting.
Yes, that is downright (POP3) protocol violation.
Cox Communications is in the process (if not already done) of converting residential Cox email addresses to Yahoo.So Yahoo's policies will now govern Cox email addresses.This happened years ago with Verizon going to AOL.