At least on BGW-320's, which are the modern routers for ATT fiber deployments, it keeps flows in the NAT table even if you use it in passthrough mode, although

At least on BGW-320's, which are the modern routers for ATT fiber deployments, it keeps flows in the NAT table even if you use it in passthrough mode, although it doesn't actually do NAT, so you end up still being limited to 8192 sessions (not that that's typically a problem, but it does mean you can't e.g. run a big web server through your connection, and you can't use 3p fiber modems without some hacks).