warn people mentioning channels or teams with lots of members
Currently you can mention a team or channel and everyone in the team or channel will get a notification. This can be seen as spam in large teams.
As its not always clear how many people are members of a chanel or team, giving a warning that the action someone is about to take will notify lots of people, before they post, will help them understand if thats really what they want to do.
In big teams, this is huge issue. Every time someone new joins, you need to educate them. If Teams warns them, they will have an opportunity to learn without having to disrupt everyone's work.