They wake up every day and try to decide how to lose users. From killing things to enshittifying them.

IIRC, POP3 downloaded all emails to local device. If you have more than one device, the first one gets it all and the second or more see nothing.

IMAP kept the messages on the server so there was one source of truth and multiple devices could access the same emails.

Most email clients support both. There are probably some people still running old POP3 clients, or they set it up one way and never bothered to change.

Not sure why gmail would care. Code's been working for decades. Unless there are security considerations not worth mitigating.

Good, nothing of value is lost.