The old GT3 Porsche had to be removed from the store since Porsche requested that when they introduced their new GT3 model. It's only listed so people that bought it back then can still use it.
Typically 4 new tracks and a similar amount of new cars come out every year. Interlagos and the new F1 are likely to be the next ones plus an ADAC 2021 car pack. AMS2 is the game to go for if you want the widest range of official single seater cars past and present; RRE single seaters aren´t very popular in multiplayer mode so I guess the developers focus more on tin top cars as people will buy them.
Not really. There are Citroen and RUF GT3 which are hidden but still can be used. The reason 2017 Porsche is not hidden is to allow people buy liveries for it.
I'd say F4 and FR3 are pretty popular. Maybe not lately, because it seems ranked championship servers took most of the players. That said, the reason more tintops are added seems to be because there's always new officially licensed content to add every year. There's not much to add in terms of single seaters, since current offer covers all the bases already and any new addition will basically cannibalize the old offer and make it obsolete. That's basically what will happen with X-17 when X-22 gets released. But at least a huge change in technical regulations warrants such an addition. New F4 for example is almost the same to the old one except for the halo, so the one we have is still a contemporary race car and there's not much reason to replace it.
True but in RRE there is a gap for historic single seaters. That´s where AMS2 and Rfactor havve excelled as you have older gen F1s almost going back to Grand Prix Legends content. RRE only has the 90s F1 cars so there is room for 80s turbo cars, 70s mobile coffins and earlier Indy Cars, to give just a few examples. Would they sell well and be good to drive though?