This landed in #3073, pretty much exactly as proposed. In the admin panel's "System DMs" tool, put a single * in the recipients field instead of user IDs, and the message goes out to every user on the instance. Bots, the system account, and deleted or disabled accounts are skipped.
It runs as a background job, so you can follow its progress (and cancel it if needed) from the admin jobs list.
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Comment by Hampus
*in the recipients field instead of user IDs, and the message goes out to every user on the instance. Bots, the system account, and deleted or disabled accounts are skipped. It runs as a background job, so you can follow its progress (and cancel it if needed) from the admin jobs list.