It's not THAT old.... but in general component availability and the march of time has made Allen-Bradley Universal Remote I/O modules out of favor and out of production.
1747-SN Series A couldn't perform block transfers, so don't use them if you're going to scan analog modules.
1747-SN Series B will scan nearly anything.
1747-ASB Adapters can control up to 30 slots of 1746-style I/O.
And then you get into the broad world of FLEX, Block, and other I/O platforms for AB Universal Remote I/O.
And that's before we get into the newer stuff on DeviceNet.
As attractive as the 1747-AENTR is, you can't control it with an SLC CPU, only a ControlLogix. It was a begrudgingly built product to fit a small number of retrofit and staged migration applications for strategic customers.