In relation to this topic here and this one as well, I suggest mirroring ‘PM User’ from the flag button to a link under “Reply as linked Topic”. Putting that feature under the flag button is not intuitive because ‘flag’ implies that something is wrong with the post and the flag button should only be clicked in that situation where Things Are Wrong And People Must Be Alerted. When I used the ‘PM User’ option under flagging today to suggest something to a user to prevent a conflict from happening, I thought it was going to be an anon message strictly to anon-suggest something so a moderator wouldn’t need to (ergo what the entire system is for in the first place).
The idea that one extra link is going to be clutter on the main ui (and in a spot where there is already a link and for a user could also be the spot to link to the PM in question just like a linked topic) does not make sense in this case, when a useful feature is hidden under a button that is (in most circles) used to alert someone to rule breaking, conflict, spam and trolls.
Furthermore, I would suggest that perhaps the PM-User option under the flagging button also alert staff, have it logged or maybe put a warning or notice of some kind where moderators are made aware that a user was PMed by another user in relation to a problem that the flag button is there for. The main reason is because this allows users to suggest things such as "Can you edit your post to our current posting standards? or “You forgot to fill out something which will delay this” or “please don’t feed the trolls” but also making a moderator aware something is going on or at least logged in the system to keep an eye out or at least know a user was actually warned/suggested by another user. That can help determine if things need to be escalated if a situation becomes worse. It also leaves a proper paper trail.
My suggestions are asking for consistency with previously-applied context that is well-known in internet forum ux. It currently makes zero sense, and I am not intending to be hyperbolic with that statement. Putting a benign feature under a button used to alert moderators to suspicious, malicious or rule-breaking activity does not make sense.