If you are not currently using WordPress credentials to sign in, you can safely ignore this post. Otherwise, read on...
Wordpress.com has retired OpenID support for blogs created after January 2, 2016. Blogs created before then should still be usable as OpenID credentials, but even then, the support has been getting more and more brittle over the years. In particular, if you have a custom domain for your wordpress.com blog, you can't use that blog to sign in to Stack Exchange. Why? Not entirely sure, to be honest. There's a redirect somewhere that's busted, and as far as I can tell the issue isn't on our end at this point.
With this in mind, we're removing the WordPress button from the login and signup UI to avoid even implying that WordPress is a viable option for signing in if you haven't already:
If you are currently able to log in using WordPress credentials, they will continue to work for as long as WordPress supports them. To log in with your existing WordPress credentials, enter your blog URL into the freeform OpenID field. I strongly encourage you to consider adding another credential as a backup option as well, or at least make sure you have a valid email set in your profile to enable account recovery.
These changes took effect on Monday, November 21, 2016, and are now live.