The message is real. Its content matches the post Support for OpenID ended on July 25, 2018 by a Stack Exchange employee.
It was not sent to all participants, only to those whose login method is being phased out. Another SE employee tweeted:
Heads up: we're about to send an email out to about 120,000 users who only have an OpenID to login to Stack Exchange. We'll be removing OpenID support from our code entirely in 30 days. This is for .NET Core, testability, and our sanity.