Let me argue please. We might think it's a simple habit to take, the problem is that it works everywhere on the web, so it's hard to take this habit because I lose it as soon as I go elsewhere.
Please, at least then remove this bug effect if you do as you do everywhere. Since this change I see that the number of mentions decreased on my site, which is open to people from 18 to 89 years.
You ask how the mentions should work in messenger. If you use the mentions in a live discussion, it is necessarily because the number of participants is important. You will not do it if you are 5. If the number of participants is important, the discussion runs very quickly. If you force me to take my mouse in hand every time, excuse me but then I was faster to manually write the entirety of the nicks. Remember that predictive research is done to save time first. Do not forget that people do not have a mouse, it's worse they use a touchpad!
For the rest the way the mentions in Rocket-Chat work fits me very well. And uh ... notice that there too we can do enter.