Mentions for older usernames that used to contain non-allowed characters do not work with new sanitized slugs

[Clorith]

The mentions plugin that auto-links usernames when prefixed with @ is struggling a little when presented with old-style usernames that used to contain a period in them.

The period is identified as a stop, and only the part before the period is treated as the username, and using the sanitized version of the username without a period, as redirected to by profiles.wordpress.org, is not recognized as a username at all.

See the user henry.wright as linked to in ​https://make.wordpress.org/core/2022/01/04/a-week-in-core-january-3-2022/ (manually linked to with the sanitized username for now, hence the different visuals of the link).

The use of @henry.wright would only link to the user @henry, and leaving .wright as plaintext, and the link would go to https://profiles.wordpress.org/henry/.
Visiting https://profiles.wordpress.org/henry.wright/ redirects you to the sanitized version at https://profiles.wordpress.org/henrywright/.

[Otto42]

The @ links you use in posts should always be to the correct nicename, not to the username.

When we standardized this (such as a "standard" that it is), then we used nicename. So, always @ to the same thing as in the profiles link. Or just use exactly what it says on that profiles page: @henrywright on WordPress.org and Slack

While using the username will cause the redirect, that's just because we're nice like that and have such redirects. The canonical name is henrywright in the both the profiles URL and what you should @ to.