all KnownSyntax clearly that username should be blocked when such an extension is enabled ?
staion why would you need to mention all users in a post when they could just Follow the thread
That's why I'm not sure this feature is useful. But there's still a difference. People who follow are interested in the future of the discussions. Some may not be interested if they are not needed, hence mentions. The @all would act the same way as putting every name in mention in the post. The @all keyword could also just be reserved and shown in another colour, just to show this concerns everyone, even if there's no notification feature at all.
staion letting this feature be used to any user will be catastrophic
That's why I would use permissions on that part.
For a use case: I use Flarum to connect beta-testers of a game I'm working on. All news are posted on the forum. For the most important ones, I do a newsletter in MailChimp with a link to the forum. Having a @all feature would allow me to do that quicker, and also remove the headache of maintaining a mailing list. Of course an admin "email everybody" would also help.
luceos and/or mention groups, eg mods or admins or game masters
This could also be useful ? Ideally this could all go in the official Mentions extensions, but for third-party extension I would do that separately !
I notice there is no possibility to define slugs for groups. It could be nice to define slugs for each group, block these as usernames and allow to @group directly.
I can't think of a use case for that at the moment... It may be useful to reference a group (but not with notification). This should not be used by new/standard users just to call for @admin. When a mod is needed, that's usually something that goes through the flag system.
