Lanefan
Victoria Rules
If the NPCs in the tavern were the only possible source of this information then sure, one can't blame the players for not interacting with what otherwise might have seemed a bunch of randos in a pub.Again, I refer you to the example that I was explicitly given by others who favored an old-school sandbox experience: the "you didn't talk to the one-eyed [or one-armed, or various other maimings] man, so you never heard that the slimes in the mines are weak to lightning but divide when struck by regular weapons, which means your death at their hands/pseudopods is entirely on your head." That's not an exact quote, of course, but it covers the core points: (1) the players were just supposed to know that some NPCs in the tavern were necessary sources of information; (2) failure to interact with the one and only source of that information is construed as the players' mistake; and (3) any deaths/losses that result from failure to interact with that source are thus earned by having made that mistake.
If, however, the info was potentially and relatively easily available from various sources but the players couldn't be bothered with info-gathering* and instead just plowed on in, then I'd say their failure is on them.
Most such situations fall somewhere between these two examples - the info is available but there might be a bit of player-side in-character work required to get it...or, as was the case in your example, painful trial and error will do as well.
* - I've DMed these players in the past. Roaring fun all round. Lots of rolling-up of new characters, though.

You're right on this one. There's a fine line between making sure something is mentioned in narration (or boxed text) while not pushing it front-and-centre. Which means in practice IME giving a broad overview narration first and then further detailing only the specific elements the players ask about, if any.I have been told, over and over, that the sandbox GM must not manipulate the players' choices. But in the very act of giving detail to thing X and not giving much detail to thing Y, that can manipulate player choices. "Oh, this is the thing the GM wrote a lot about, it must be Important" is a perfectly natural thought for many players, of any style. Likewise, things that don't get any description at all are at risk of being written off as unimportant or non-interactable.
I'm fine if they miss things through not investigating further (or at all), even to the point of their being unable to progress further. I'd prefer that.......This is...kind of essential to the trilemma. The first path, giving no bias by (almost) never prompting, only waiting for players to act--but then the (from my perspective) very high risk of players never even considering something due to lack of prompting.
.....over this.The second, accidentally manipulating player action toward the things you prepared and away from their own choices/creativity.