Yea in the end Publishing has always been a negotiable thing... There are cats like Ken Lewis who interpolate samples that get publishing and co-pro credit... There are people who helped finish the verse and they get publishing...

it all varies and unless we saw the split sheet in older sessions we dont know...

Traditionally people who are "work-for-hire" didnt get publishing... session players, background singers etc... even if they contributed a riff or 2..