mautrix
Bridge discord messages sent with @silent as m.notice
Bridges discord messages marked as @silent as matrix notices.
Notice the bell icon in the corner
As far as I understand an empty m.mentions still sends a notification, just not a ping. While a discord silent messages won't send any notification even if explicitly enabled in the room. Idk if this is realizable some other way than a m.notice.
m.notice doesn't necessarily disable notifications, it's just a marker for automated messages. Bridging @silent properly probably needs a new MSC to define a new field to signal silent messages and a corresponding push rule
I'm currently just using this branch on my server so I'm totally fine with this never being merged (you're right, an MSC would be the right way to go about this).
I was just thinking, however, whether bridging messages that are marked as @silent and sent by a bot as notices would make sense, as these are clearly meant to be automated + not interrupting/notifying users.