What happens if a 3.3V supplied IC pull-up to a 5V with resistors?
Since the pull-ups are to 5V on a SHT21 powered by 3.3V VDD, the voltage on I2C pins will exceed the nominal maximum of VDD, and most likely will exceed the absolute maximum rating of VDD+0.3V, after which there might be an internal protection diode that can push current from I2C pins to VDD, and if VDF is lightly loaded, the VDD voltage can rise somewhat too. VDD nominal max is 3.6V and absolute maximum is 5V.
If clamping currents goes into IO pin, that is not considered nominal operating conditions. Chips don't necessarily work when used out of nominal operating conditions, and may not be immediately damaged, but could be in the long term. Exceeding absolute maximum ratings is likely to cause damage. Anyway the measurement accuracy can suffer if the VDD supply fluctuates depending on I2C bus traffic.
Releasing a product that has a known issue that can affect long-term reliability could be considered unprofessional, and depending on situation, it could happen that somebody debugs a misbehaving product and finds out that there is a hardware bug.