Hmm, it appears that the top byte on arm64 is only ignored if TBI (Top Byte Ignore) is enabled.
I don't think pointer signing requires TBI though. Pointer signing uses the PAC instruction to sign a pointer, and the AUT instruction to verify and unpack the signed pointer, but in its signed/packed form it is not a usable pointer. So actual addressable pointers need not support non-canonical addresses.
