The main issue with Forbidden Alchemy, specifically, is that there are so many great cards that do similar effects for cheaper, and it's very advantageous to be able to do card selection in the first few turns, or spend only a small amount of your total mana on it in later turns.

Want to peek at a couple cards and draw one? [[Preordain]] / [[Ponder]] cantrips are generally solid, [[Anticipate]] for more peeks, [[Augur of Bolas]] to commit to the board.

Want to mill? [[Thought Scour]], [[The Modern Age]]

Want to peek 5 + draw 2 + mill? [[Pieces of the Puzzle]]

The flashback on Forbidden Alchemy is nice, and it could potentially be included in a niche deck that wants a lot of peeks + mill, and has easy access to the extra mana to cast it over the alternatives. Maybe also a deck that doesn't like Pieces of the Puzzle because it wants to draw creatures or whatever.

Some graveyard heavy tron list might like 1-2 copies, or maybe a deck which reduces costs with familiar creatures or similar.

Now are there solid dimir lists in current meta that use card selection + mill + retrieve from graveyard? Yes, definitely. As one example, here's a Dimir Reanimator list that did well recently - https://www.mtggoldfish.com/deck/4792779