I've seen a Sentra SR in the fleet, but, like Closingracer, not the Altima SR. The SR slots between the S and SV in price, so I'm guessing it's Nissan determining the mix and not an issue of cost.
Glad to see the mix to trims, though. A 3.5 SL was my third choice for my last rental (first choice was a brand new 2017 Fusion Hybrid followed by a Fusion Titanium).