It seems it only blocks the transfer when you transfer to a new object in the object settings (it is the new object that must have the unique parameter exclusion set so as not to accept that parameter).
It has no effect when transferring between existing objects.
I am not sure this was always the case but I may be mistaken as I rarely use it.
Versions 6.5 to 22
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision M6800 - i7 4700MQ @ 2.40GHz, 16GB RAM, AMD FirePro M6100 (2GB), Windows 7 64bit