TL;DR: Signing up or logging in via OpenID is no longer officially supported from any of our sites.
Stack Overflow was an early and strong supporter of OpenID. We built our sign-up/log-in flow around it. We were idealistic and had high hopes, but these hopes weren't realized. Over the years people have wondered if OpenID is dead. We've had to remove support as OpenID providers pulled support or shut down.
The time has come to part ways. The reality is OpenID support has created a ton of complexity in our codebase and the number of users actively using OpenID simply doesn't justify that cost. Users have spoken with their actions. You prefer Google, Facebook, and Stack Exchange (aka email/PW) based account auth.
Of our 9,813,747 accounts around one-tenth of a percent are actively using OpenID (defined as having visited a site in the past 12 months). If you include all the inactive accounts it is still less than 2.9% of all accounts.
So, what is the plan?
DONE - Remove the ability to create a new account with most OpenID providers
DONE - Add the ability to add email sign in to a current account
DONE - Convert Launchpad (used on askubuntu.com ) to OAuth
DONE - Remove creating new accounts with Steam from Arqade (Steam removed support for OAuth. Only 454 accounts that use Steam have visited Arqade in the past 6 months.)
DONE - We will email all users that rely on deprecated OpenID credentials and request that they add a new provider to their account
DONE - Run an announcement banner for any user who signs in with deprecated OpenID credentials urging them to add a new provider to their account
DONE - End support for logging in via OpenID
DONE - Remove openid.stackexchange.com (note: only 0.2% of authorized sites in the last year have been non-Stack Exchange sites.)
Obviously, if your account relies on OpenID, then make life easy and go now and add your Google or Facebook credentials or set up email/password auth.
If you've forgotten what credentials you use to log in, then click this link to refresh your memory. If you don't see one of the three account types shown in the image below (Google, Facebook, or Stack Exchange) then click the "add more logins" button to add one of them to your account.
Remember, SE OpenID accounts are really email/pw accounts handled through complicated OpenID code paths. They will be migrated to a simpler email/pw only code path as a part of this change. No extra work is required on your part.