Fearless In Quarantine said:
Zamasu is a rare being though not an unusual Kai with that kind of strength.
Even though Shin seemed awed by Zamasu's strength, Beerus was never so impressed, which would bring into question if such as Shin-jin had ever existed in Beerus' 75+ million year tenure as a Hakaishin. With that said, it seems very improbable that in all that time, not one Kaioshin from within the multiverse surpassed SSJ Gohan.
no mortal can go to the Kaioshin world.
If anything, wouldn't that support the idea of Beerus taking precautions in making sure no Shin-jin could break it, yet still open to mortals? Even though the multiverse could still produce mortals far above SSJ Gohan, Beerus treating it as inconceivable one could kill Freeza would suggest it's only a recent trend and thus not a factor in his seal.