Yes, and it still is. I have been using Gmail since 2004 and I very rarely touch the web or mobile UIs. 99% of my interactions with it are through SMTP and IMAP. I don't think that Gmail has fundamentally changed in this sense, it has just gotten very popular.

The various incarnations of their chat services is a better example. Gtalk used to have great XMPP support, even federation for a while. All remnants of that are gone now so I had to stop using it.