You'll notice that nowhere in that block is the word "MUST" or "SHOULD".

This has had consequences.

On MacOS, loopback is defined as 127.0.0.1, period.

On Windows, loopback is defined as 127.0.0.1/8, so you can ping 127.0.0.1 thru 127.255.255.254 as per usual /8 network allocations.

Linux is....different. Linux has opted to include 127.0.0.0 as a pingable address for loopback.

Yes.

.0