So, it is presumably possible for a DM to decide something isn't allowed in their setting. Or at least not to include it in the base writeup. It would, I believe, be acting in bad faith for that DM not to make that information available to the players--I agree this is highly unlikely, shading to implausible, but A) it would be acting in bad faith and B) it seems some people commenting on this thread have in fact encountered DMs behaving exactly that way. It would also arguably be bad faith to pretend to listen to the player who wanted to play something "off the list" and to have no intent of ever saying anything other than "no"--this seems slightly more plausible to me. Making the list available to the players, being willing to explain your decisions t your players, being willing to work with your players to find things player and DM can enjoy in the campaign--those are all good faith actions.The sticky wicket here is the idea of DMs acting in bad faith. This is, to put it simply, a very difficult thing to picture in the context of this discussion. There are lots and lots of ways for DMs to act in bad faith (excessive fudging, railroading, illusionism, bait-and-switch, gotchas, "rocks fall," magical realm, etc. etc. etc.), but leaving elves or warlocks or whatever out of a campaign setting simply isn't one of those ways. If your thesis is some flavor of "DMs acting in bad faith are bad," again, no arguments here—but curating a setting or a ruleset isn't bad faith, and it's really really hard to imagine how it could be in a situation that isn't cartoonishly absurd.