I read almost everywhere that Sage was originally one of the most, if not the most, powerful agents in the game, and that a lot of nerfs have made her much weaker. But exactly how powerful was she in the beginning? Like, how does her abilities compare between then and now?

I'm relatively new to the game. I have watched several old videos, but I haven't found a lot of difference in her abilities from then and now.

Personally, I think she's a pretty solid agent to play with. Sage was the first agent I played, and even with all of the agents unlocked, she's still my top pick in most of my games (I prefer a more supporting playstyle).