Ok so if you switch to old scoreboard, then Hold in tab the entire time while switching back to the new one...theres no HUD. Ammo, Health/armor, reticle, everything...it's all gone. Not sure if this is widely known, or if this would be considered a "bug" or whatnot. Just...yeah.

Perhaps this could also be considered a good training tool? At any given time, a person would no longer be able to tell how much health, armor, and ammo he/she has. Also, no reticle meaning aim gets better perhaps? (Or fucks it up I suppose)....

Sounds like a bug. I doubt TrotSkie is going to do an awful lot more with PF though.

should come back after you restart the game

Playing without a reticle can help you develop a better sense of where you're aiming without focusing but the efficacy of that sort of practice is debatable. Removing the hud would make you more conscious of what items you're taking, but I don't really think it's very useful 'practice' since it's rather trivial awareness.

You're better off focusing on developing awareness of useful information: opponents positioning, habits, weapon usage, how much damage you're doing in fights, etc.