a) Is there room for games to simply mention there are undisclosed themes for games where the GMs want to keep elements of the game a secret? It's not something I do much of these days, but I know there are some games that like to have surprises. I would hope that GMs at least side on disclosing things by default, particularly with content that is likely to be objectionable.
There have been some instances where this hasn't happened. I mean, I don't think the GMs of those particular games were trying to give people a bad time, just they were keen on some heavy plotlines and wanted to go for a surprise twist, like characters recovering memories of being tortured. Or character sheets weren't provided until right before the game that had some difficult content, with no room for the player to opt out without letting people down.
I was going to suggest amending the procedure slightly to give examples of what should be notified to the players, such as: graphic violence, sexual coercion (or underage sex), roleplaying content that would need an R rating on a TV show, putting the players in a stress environment (eg very crowded; very dark; absence of food, water or restroom facilities; unusual temperature.)
I'm not saying don't have heavy games that explore difficult themes, because there's definitely a market - but give players enough information to go in mindfully, and opt out procedures for them to exit if it turns out to be more difficult for them than they expected. (There are some techniques like Cut/Brake, X Cards, Lines and Veils which are worth looking up if this is the kind of game you want to run.) And plan for a longer debrief phase of your game, as well - people can get surprised by Big Feelings in larps and taking the time to transition them out of the game space at the end can help them a lot.