Reddit enhancement suite would do manual, single user tagging and the Masstagger browser add on would do… well, mass tagging.
I used it to show me when people I interacted with made more than 50 posts/comments in places like r/conservative or r/thedonald. It would also link you to the comments so you could see what they were saying there.
I found it helpful because there were times when I found people undermining concepts like cultural pluralism and participated in those subs. I knew where they were coming from and what they were trying to convince readers of (nothing good).
Several times it helped me effectively argue against white supremacists.
As long as comments are public, which I think is the point of sites like Reddit, lemmy, and kbin, those types of plugins and info will be available.