Center tap of transformer for Ethernet interface
It depends on the output type of the PHY.

If it has open-drain outputs then it requires the center tap to be connected to Vcc. If it has push-pull outputs then the center tap has to float.

There are probably silicon-level trade-offs like area vs. power vs. speed vs. EMC vs. manufacturing process etc. that lead to choosing open-drain or push-pull configuration.

If you connect the center tap to Vcc for a PHY that does not allow that then in the best case it may just work, or it may only work for a particular speed, or it may not work, or you will blow the chip.