Here's how you can download older versions of files in a game from Steam using SteamDB to navigate the game's patch history. The example here is reverting Skyrim SE's 1.6.640 patch so we're back to the 1.5.97 version of SkyrimSE.exe.
You can find all of the available depots for a given game by looking up its appid as follows: https://steamdb.info/app/489830/depots/ - 489830 here is the appid for Skyrim SE.
First, you have to find what changed, so click the History button down the left side: https://steamdb.info/app/489830/history/
That shows the last change - Changelist #16107407 at the time of writing - as happening for depot https://steamdb.info/depot/489833/ which is Skyrim Special Edition exe and contains just SkyrimSE.exe.
If you click the History button on the left, you can see all the changes released for that depot: https://steamdb.info/depot/489833/history/
As the first Manifest says, SkyrimSE.exe was changed earlier today.
To grab an earlier version we need to find the one we need. 1.5.97, the version we want to use, was released on 21 November 2019. So scroll down and there's a change on 20th November 2019, the day before. The manifest for that change is 2289561010626853674.
With all that info: appid 489830, depot 489833 and manifest 2289561010626853674, we can download that version of that manifest from Steam.
Type steam://nav/console into your browser, hit Enter. Steam should now have a Console tab. Go into that tab and enter: download_depot 489830 489833 2289561010626853674
This is the appid, depot and manifest within that depot we want to download. You'll see something like this but your paths will be different because I'm on the Deck:
] download_depot 489830 489833 2289561010626853674
Downloading depot 489833 (23 MB) ...
Depot download complete : "/home/deck/.local/share/Steam/ubuntu12_32\steamapps\content\app_489830\depot_489833" (1 files, manifest 2289561010626853674)
If you go to the folder shown, it'll have the right version of the files in that manifest. In this case, it's just 1.5.97's Skyrim.exe:
(deck@Deckard depot_489833)$ pwd
/home/deck/.local/share/Steam/ubuntu12_32/steamapps/content/app_489830/depot_489833
(deck@Deckard depot_489833)$ ls -al
total 33964
drwxr-xr-x 2 deck deck 4096 Sep 20 17:17 .
drwxr-xr-x 3 deck deck 4096 Sep 15 15:54 ..
-rwxr-xr-x 1 deck deck 34769792 Sep 20 17:17 SkyrimSE.exe
Copy that file to where you want it and you're done.