I'm not sure that's possible.
Thus far we've not seen a single cross-gen game where a fundamental, gameplay-changing mechanism is in one version but not the other. It's only been changes to the graphics, grid sizes (in racing games and in theme parks), audio, and DualSense support.
Dynamic TOD/weather is such a mechanism. It doesn't just look nicer, it changes how the game plays. Imagine playing an FIA race (or any Sport Mode race) where the sun sets for half of the grid but not the other half, or where two thirds of drivers aren't dealing with a spell of rain halfway through the race - and a drying line. You'd literally be looking at a PS5 being a disadvantage; hardly a console seller is it?
Of course it doesn't have to be in Sport Mode at all, and Sport Mode might not even be cross-gen play (though if the game is, it should be; how are you supporting those 110m PS4 players if they can't play it? Also PD doesn't exclude players from Sport Mode for not having a T-GT, so why would it exclude players for not having a PS5?), but it affects single player too.
On PS5 you might have the 24hr of Le Mans (or not, if Motorsport Network plays an exclusivity card) in your career mode, with full day-night transitions and weather, just like you had in GT5, in 4K60. But on PS4 you get 24 hours of racing in whatever preset conditions and you race for 1,440 minutes with the sun up (or down), just like you had in GT4, only with 20 cars instead of 6...
You don't even have to imagine that hard to see how it'd create two completely different single-player games. No proper endurance racing (yet again), no day-night events (or dry-wet events) anywhere in the ladder, no challenges like that Special Event in GT6 for B-Spec Bob where it started to rain 7 laps into a 10 lap race, any plans for licence tests/missions involving TOD/weather.
As far as we're aware, that can't happen; the gameplay itself must be the same between both versions. That means you can't leave it out from one version but not the other, so it's either cut entirely, running at lower resolution/fps on PS4 (unlikely), or PD makes it work at 1080p60 on PS4 (very unlikely or a console-killer).
Imagine though that it is possible and take it a step further. What happens when you have people who aren't like us and don't sit on racing game forums all the time who stumble across GT7 and think "hey, GT used to be cool back in 2004, let's give it a bash". Dynamic weather, dynamic TOD, new in-depth physics engine, awesome. So they pick up their PS4 copy from the PS Store, download it, and get into a game with literally none of that.
Now we're into PCARS3 territory where they feel absolutely ripped off, because it turns out that GT7 doesn't mean GT7 if it's PS4 GT7. Every single piece of advertising they've seen is from the PS5 version (because why wouldn't you promote it with the best version) and the asterisks all over the box/store page haven't hit a relevant part of their brain. "What, so I have to throw £400 at a console and THEN another £10 to upgrade the game to get it to do the things it was advertised to do?" - and no refunds, because it's a first party game and they played it for a fraction of a second. Cue the forum threads full of people accusing Sony/PD of being scammers, and GTP too (because we are Sony/PD to a lot of people for some reason, and also we'd be pointing out they got the PS4 version).
And then there's the other side of the coin. Promote the game with the PS4 version at all and you'll have PC4/FMnot8 fans and forums bringing up how terrible it looks for a 2022 game. People are still posting the Standard model Suzuki Alto Works from GT5 to show how crap Gran Turismo is, and that was eleven - count 'em - years ago, not to mention GT6's ropey alpha effects from 2013. Do you think full daylight 24h races, or 30fps rain/fog/night with jaggy shadows, will get any less or die down sooner?
