There is a small mention in the README_6B.txt file.
0.6B is NOT compatible with version 0.5Z28.
0.6B CAN play most version 0.5Z28 replays.
There is no backwards compatibility with Z28 hosts. 0.5 and 0.6 versions cannot possibly connect online because there are too many changes. Z28 hosts should all upgrade - I can't think of any reason to stay with Z28.
Good work! Tho I found an issue. When playing on the BL1 Car Park in the open config, (was trying to see if that barrier in the entrance is there or not) if the car turns too far in either direction the game crashes. This seams to happen to me in any car. This is the only track it has happened on so far. Anyone else having this issue?