PhutilBitbucketAuthAdapter still uses OAuth1 (https://github.com/phacility/libphutil/blob/4206849bb05b60f536a1c78e33adee68dac67aa9/src/auth/PhutilBitbucketAuthAdapter.php). However, OAuth1 on Bitbucket was deprecated several years ago and since been end-of-lifed.
libphutil should use OAuth2 on Bitbucket instead of OAuth1.
The Bitbucket OAuth2 endpoints are documented here: https://developer.atlassian.com/bitbucket/api/2/reference/meta/authentication#oauth-2
OAuth1 is no longer supported on Bitbucket and will likely be permanently turned off before April 2019, which would break Phabricator.
Switching from OAuth1 to OAuth2 does not require the creation or registration of a new OAuth consumer key/secret pair. The existing consumer can be used with OAuth2.
Affects all versions of libphutil (and by extension Phabricator).
Signup in Phabricator using the Bitbucket option. libphutil then redirects the browser to https://bitbucket.org/api/1.0/oauth/authenticate which is Bitbucket’s OAuth1 flow.