>>714624010
>>714623864
>>714623280
I checked the text dump to be sure.
>THEN... WHEN YOU REACH THE CAPITAL, CROSS THE BARRIER. THAT'S THE MAGICAL SEAL TRAPPING US ALL UNDERGROUND.
ANYTHING CAN ENTER THROUGH IT, BUT NOTHING CAN EXIT...
Papyrus mentions the barrier as explicitly magical, it isn't lost knowledge (at least from the monsters' perspective).

One of the library books says this:
>While monsters are mostly made of magic, human beings are mostly made of water.
>Humans, with their physical forms, are far stronger than us.
>But they will never know the joy of expressing themselves through magic.
>They'll never get a bullet-pattern birthday card...
I think you can read this as one of the following:
>humans can't do monster-style bullet magic
>magician humans are technically non-human (c.f. touhou lore)
>the writer is biased