OP here. I've been on Gmail for around 13 years and it has never reminded me of this. So I just now ran a test, sent a mail that said "attached is a copy of y

OP here. I've been on Gmail for around 13 years and it has never reminded me of this. So I just now ran a test, sent a mail that said "attached is a copy of your letter.", and Gmail sent it without comment. But, when I enable the Javascript/Standard interface, it did remind me. I use the "Basic HTML" interface and never switched to the JS/Standard interface so I never saw the reminder. I tried the Standard interface recently and really don't care for it much, though I concede it has more features, like being able to select all messages in a mailbox. I had to use that to get all my Inbox mail moved the the Archive folder on Gmail when I migrated to Fastmail.