So where do you draw the line line here? Do you roll an ability check to know that wizards cast spells or that clerics worship gods or fighters can use action surge or that gold is the currency in the city you are in? Do you need an ability check to know you can get food at a tavern and buy a room to sleep in at an inn and if you fail those checks does that mean you can't eat or sleep while you are in a city?
I'll be honest: I have no idea what this reply means, or what it has to do with my post.
I just finished saying that I no longer ascribe to the belief that players have to pretend to not know things. Given that, why on earth would I think you need to roll dice in any of those examples you gave?
In the example I used my character has a background of cloistered scholar and he does not know who one of the most infamous historical personalities on the sword coast is?
I'm not sure what this means, either.
But here's what I would say: regardless of background I bet you could think of a hundred reasons why your character would or would not (your choice!) have heard of this NPC. Therefore:
1) If you think it would be fun to pretend to not know who she is (maybe because you want the other people at the table to be surprised) go for it.
2) But if you don't want to, you shouldn't feel any pressure to.
If your DM simultaneously doesn't want players to use out-of-game knowledge
and used an NPC straight out of best-selling novels, then...man, I just don't even know what to say. That's just crazy. That's just as dumb as using tall, green, warty monsters...and maybe even referring to them as "trolls"...and then expecting players to pretend they don't know to use fire.
I mean, FFS, how easy is it to rename Valindra to something else, or give trolls stocky builds and brown fur and
then don't use the word "troll"?
As other posters, and the Angry DM, have said...metagaming "problems" (if you think they are problems) are 100% the doing of the DM.