First, citing extraplanar adventure as an example is a corner case. Second, I have a hard time believing Barovia is completely lacking in either "a library, scriptorium, university, or a sage or other learned person or creature." Finding out what or who and where those things are shouldn't be all that difficult for a sage.
I don't agree the game needs to be protected from bad DMing, especially not with a boring redesign like this.
(edited for spelling)
First, setting a campaign in a pocket dimension like Barovia, the Astral plane like Spelljammer, or a different plane of existence like Avernus are what happens in published mods.
Second if you don't care about the logic of how the world works, more power to you. I couldn't take a game like that seriously and wouldn't want to play in a long term campaign that does not. If I'm in a city my PC has never heard of, why would I have a contact there if I have a criminal background? It would make no sense.
So I think it's bad DMing to twist logic into a pretzel so a PC gets a minor benefit.