To my understanding, at no point did they completely re-write the graphics engine, it's all been incremental changes. That's the point here; when you update something incrementally you always have some legacy code in there. There may no longer be any code from the original incarnation of the engine, but the incremental approach always means it's behind the times because you build in steps, which takes longer and usually requires some kind of workaround to make some newer things work with some older things, even if only temporarily.
They haven't started over from scratch, and it shows. That's what I was getting at.
It's not a slight, by any means, either. It's still a pretty good looking game, all things considered, and the incremental approach was a smart move from them for exactly the reasons you mentioned; it's easier to keep the whole player base on the same level that way than it would be if they introduced an entirely new engine that left behind the older machines.
Still uses the same format .dat .mip .lp files they used as far back as 1999 with GPL and the older IndyCar 2 and Nascar 3 / Legends.
I believe you are correct that there has not been a 'complete re-write'. That's why there are still hard coded limits to cars in a series / session.
I also do not care for all their fake series with the unrealistic salad of old cars and non-existent cars. Their IMSA series is pathetic. Non-existent C7 DP, no LMP2 / DPi, no Acura GT3, no Lexus GT3 and very few of the tracks. The LeMans series is missing tracks, the LMP cars are dated, no Bentley, no Aston, no M6, no 650 or 720, no C7 GTE and they substitute a bunch of dated or non-existent content for this as well.
Some of the most complete and best multi-class fields are in rF2 and RRE at the moment.
The physics / tire are horribly botched. Doesn't really drive realistically and there doesn't seem to be proper tire cycles. They go from cold grip to hot and slippery (backwards). Real tires cycle in and grip up as they warm then cycle through different phases of grip as they wear. Not their thing.
The ffb has seemed to have gotten better over the last couple years but it's still meh comparitvely...
The new damage system looks interesting. Whether they implement it in a useful way remains to be seen. I don't think most on the service would be able to drive the cars with the new damage as shown so far, and finish any session.
The system and mmo environment is great but everything else is lacking.
My 2nd least favorite sim next to AC. Can you tell?
* Edit *
https://postimg.cc/Wtpq2j7V
You can see in the hex of a .dat file, the .3do .mip .stp .ptf .lp and whatever other stuff. All the same file formats they've used for nearly 25 years.