Yeah maybe not “blasphemy”. It can absolutely be done well, but the mindset of houseruling in 2e is very different from the mindset of 5e houseruling. Like in 5e, the thing going through your head when you houserule is “the 5e rules don’t cover this situation appropriately so I need to try and come up with something that feels fair and in the spirit of the system design” whereas in 2e the system feels far more complete and has fewer holes.