It's really interesting how much more antelope species Chester used to have, Addax, Lechwe, both Oryx species, Blackbuck are all gone now. I actually didn't even remember them having Addax at all! (This isn't actually even listed on zootierliste interestingly enough)
Now they currently only have one antelope species on show, Red Forest Duiker. They have five other species which are currently off show due to the Heart of Africa Development: Kirk's Dik-Dik, Sitatunga, Eastern Bongo, Blesbok, and Roan Antelope.
After doing a little bit of digging, it seems Chester did in fact never have addax! They had plans to get them around this time but fell through (foot-and-mouth disease to blame?) and the enclosure was repurposed for cheetah.
@Fallax This map is quite the historical document if there is an Addax icon (right in the middle) and yet the zoo never even had the species. Remarkable!