I thought Nic said they were rehearsing Abacab at one point? Even if I am recalling that correctly it dosen’t necessarily mean that Abacab made it into the final show. One thing to keep in mind tho is that while it’s been 35 years since they played abacab, that was only 2 tours ago for this lineup of Genesis (3 if you include CAS). And of all the “long dead” songs that Genesis could revive, Abacab is pretty likely because it was a hit (peaking at #9 in the UK & #26 in the US)
You are right, everything is on the table and any song could be revived. I was always surprised they dropped Abacab, speaking of openers it could be one, it's an obvious live number, it was a minor hit and fans, seems to like it, not me, I've always thought there's nothing to it and it wore out its questionable charm pretty quickly. One redeeming quality was Phil's vocals, he was in top form and could sing the telephone book back then. It's different now of course.