I think the best definition of "sifter" is a card which increases the frequency with which you play (meaning more "is in your final hand state" and only has real trouble with Tunnel) your "good" cards relative to your "bad" cards without trashing or setting either aside.
So for the obvious cases, Survivors is obviously a sifter - if you see good cards you leave them, if you see bad ones you drop them and don't play the bad cards this shuffle.
Warehouse - draw more cards keep good cards, discard the lousy ones.
Inn - draw more cards, keep good cards, discard lousy ones
It also excludes pure draw. Smithy makes you see your golds more often, but also your coppers. The relative frequency of Gold vs Copper doesn't change (or Gold vs Estate). Scout would be pure draw and when Scout is worth getting (most often for "free" off a spare Iw or Prssn play but not even always then), you want it as draw. E.g. if I'm doing Xroads/Nobles, I want scout as draw. If I'm doing Iw/Tournament/Scout, I want my Scout to draw the Provinces/Duchies so I can play Province with my Tournaments. Vagrant is drawing as well, yes I have no green in my next hand, but only by dint of drawing it into this hand (Vagrant being like playing a Lab after being hit by B-crat).
This also then allows us to talk about mild sifting and anti-sifting:
Scavenger - usually a very mild sifter (discard your coppers/green/curses, be assured to hit the best card in your deck/discard next turn).
Chancellor/Messenger - an exceptionally mild sifter with deck tracking - basically it is like Survivors except for instead of looking at two cards vs rest of the (draw) deck, you are looking at draw deck vs deck. Most often this is too weak to care, but in some cases (e.g. Chancellor/Stash is an extreme case) it can become very strong sifting.
Anti-sifting is something that makes you see your good cards less often than your weak ones.
Loan is an anti-sifter. You see your good treasures and actions less often (thanks to discarding). This negative of Loan is made up by its ability to trash out your weak treasures and sometimes a very small bit by sifting past your green with late game shuffle dynamics.
Wandering Minstrel is a sifter for good actions (play them more often) and against green, but an anti-sifter for Ruins.
Venture is a sifter for good treasures against all actions and green (excepting Harem).
Sage is a sifter for high value cards against low value cards. So if you Fool's gold or Courtyard is your power card, Sage is an anti-sifter; if your power card is a Hag, then Sage is a sifter.
Hamlet this is mostly a drawing card (you don't have to discard anything you draw), but it can let you sift in some decks (discarding junk can enable terminal draw).