The magic URLs for creating a pre-filled message don’t work anymore. This is a recent regression.
For example, clicking Discourse Meta - The Official Support Forum for Discourse should open a message to @team (or at least show an error if that’s not possible), but for me, it currently does nothing besides redirecting to /lastest.
As per: Compose a new pre-filled personal message via URL
The correct URL is: Discourse Meta - The Official Support Forum for Discourse
… and it works as expected.
Uh, weird. I can confirm this works, but unless I’m seriously hallucinating 
, group_name also worked before the last beta update. (I had inserted such a URL into a newsletter-like email I sent out, and I always check these links to make sure they work.)
Anyway, thanks for correcting me 
Okay, added on my list to look into this.
I don’t think we ever supported group_name parameter as there is no source code trace for it but I added support for it regardless. Now we support both groupname and group_name parameter, if both are present the priority will be given to groupname.