From something I posted on our site:
From my experience, unless a change in a Google product degrades functionality for a large portion of end users they aren't going to publicly acknowledge it. They don't seem to have the same policies regarding transparency that other companies like FetLife do (we acknowledge when we mess up and break stuff!).
When ReCAPTCHA went down last year I looked and could not find any acknowledgement of the issue from a Google employee. This was while FetLife and other large websites tweeted about it and posted in Google's product forum. All we had to go with was speculation and helping each other out with recommendations for temporary solutions.
Likewise, this change in Gmail may never be officially acknowledged. It may be related to the arrival of Google's new Inbox product or it may be related to some sort of spam filtering they deployed into production. It doesn't appear to be specific to adult websites. Other large community websites are affected too. The result is that affected websites using plain text emails are now forced to switch over to HTML-based email. I could see a conspiracy theory in there about Google pushing HTML-based email for a nicer looking Gmail/Inbox experience. No matter what, I think it's very unlikely we'll ever know what change caused this or why.