AFAIK "open bus" is found only on these early systems with simple synchronous buses, as I believe most others will give a constant value of all zeros or all one

AFAIK "open bus" is found only on these early systems with simple synchronous buses, as I believe most others will give a constant value of all zeros or all ones when attempting to access nonexistent addresses, since the bus protocol has handshaking that lets the master know if there's no response (a "master abort" in PCI terminology.)