Forward message: hidden DMs can't be picked as target, intended?

Some of our users hide DMs to keep the sidebar tidy. Later they want to forward a message to one of those people and can't find them in the forward dialog. People you never had a DM with don't show up either, while @ mentions find everyone just fine.

The dialog only looks at your own subscriptions and skips hidden ones (open: { $ne: false } in UserAndRoomAutoCompleteMultiple.tsx, there since #26662).

Steps: hide a DM with user2, open "Forward message" on any message, type user2. Nothing. Unhide the DM and user2 is back.

Seen on 8.6.1, same code in develop.

Is that on purpose? If not, which way would you go?

1. leave it
2. also show hidden DMs and rooms
3. search all users like @ mentions do (with the usual permission checks)

I'd prefer 3, so forwarding and mentioning find the same people. 2 would already help our users though.

Once the decision has been made, I can also contribute the code for it.