I think if there is a MANDATORY side effect (Council Room / Lost City) you are absolutely obligated to make sure your opponents do it. You can’t say “well it’s on you” for the good stuff but be on their ass about attacks, that’s just inconsistent, and imo a form of cheating. You don’t *need* to check in on people about optional stuff but I think it’s nice to announce “oh you guys can get the Vault effect now” or whatever.
For optional effects, e.g. Fool's Gold, I don't think you're cheating if you abstain from pointing my options out to me (while making damn well sure I discard to your Militia). I do think you're playing hardball if you do that, and I play softball, so we should both find new opponents. (* "you" is the general you, not Chris-is-me.)
Last I checked, the tournament rules of M:tG obligates both (all) players to track the game state and ensure that only valid moves are made. I think that makes good sense. Under such a rule, a tournament official can sanction a player if they e.g. don't remind their opponent(s) to draw from Council Room. Deliberately pushing the game towards an invalid game state smells like cheating to me.
Not doing an optional thing is a valid move, though, so this doesn't obligate players to remind their opponents who leave $2 unspent that they can discard their Wine Merchant. Reminding your opponent is Good Guy Greg territory. Not doing so is maybe Scumbag Steve territory.
[...] MANDATORY [...]. You can’t say “well it’s on you” for the good stuff but be on their ass about attacks
(emphasis-by-bolding is mine.) Note the change in adjective. If what you're saying is "not reminding your opponents about mandatory effects (that happen to usually be beneficial) is cheating", then I think I agree. If you're saying "not reminding your opponents of optional and (usually) beneficial effects while reminding them of (usually) detrimental effects is cheating" then I strongly disagree. Within some subset of the gray area, the latter might be unsportsmanlike conduct, perhaps, but that's different from cheating.