Hopefully the last breaking change.

enoX should always stay stable, as it's the BIOS (in some ACPI table) telling that this device/port has this ID.

ensX means the NIC in PCIe slot X, but in your PCIe tree you can have PCIe bridges, so technically you could have multiple NIC in the same slot (what the BIOS declare as a slot), so there was a lot of breaking NIC naming changes over the years in systemd to figure out the right heuristics that are safe, enabling/disabling slot naming if there is a PCIe bridge, but just in some cases.

Also for historical reasons the PCIe slot number was read indirectly leading to some conflicts in some cases (this was fixed in systemd 257)