Jira has mentions and it’s not social media. Basically anything with users who need to communicate(?)

Tons. I think it’s a modern UX issue, and the fact that social platforms popularized an @, #, shortcut platform means even my mom (somehow) knows how to use it. It’s a stretch saying, but i feel there’s a loss for a developer/product to not take advantage of the momentum around popularized social-style tagging patterns for unrelated use cases.

Dropbox paper has mentions and I wouldn't consider it social media. Same for trello.

Many apps that have collaboration features support some type of mentions and labels to tag users.