I'd imagine the adoption of DMARC, SPF, DKIM, etc. have just functionally killed the feature.
I am happy we will no longer have to support this. Losing the POP access a couple years ago got about half of them to move, and this will kill it off.
It is useful for anyone who has a personal domain from which they'd like to send/receive without needing to migrate off Google entirely. My registrar gives a few email addresses with SMTP support. Then I use GMail's send-as to remain in gmail but still use my name@mydomain.com mailbox.