When I played Tomb Raider 1 last year, it worked perfectly on the Steam Deck.

When I downloaded it today, it didn't. It appears they switched to a different DOSbox version, SVN something, and the game is now completely broken on Linux

How do I get the old download back with the previous DOSbox version? It's not listed in the downloads section

I don't know how to get a specific old version back - without personally ardchived old downloads I don't think we get what's not offered under downloads anymore.

But I installed TR1 mid march via Lutris what still looks to be the current version (20180105). It's working perfectly fine.

According to gogdb the version number is actually the build time - so there was no recent update. It's also not using Dosbox.

It is using Dosbox. And I know it's different because it used to be regular Dosbox with the red splash screen, and when I reinstalled it today it was a different version of Dosbox that says "SVN" in its window and it doesn't have the red splash screen. All other things being equal including the specific GE Proton version I used, something has in fact changed

the Gog version in no way needs to work with the steam deck. it works on PC fine. maybe the Steam forum is where you need to be.

I don't see a reason you can't download another version of DOSbox. Just place it in the folder to overwrite the unwanted DOSbox version. I don't own a Steamdeck, but I'm guessing you just copy/paste the folder over, or similar, right? I know I wanted the Liesure Suit Larry games on my phone so I installed a phone version of the engine & put all the games on my phone & the engine ignore what it didn't need.

Tomb Raider didnt get uldates since 2019. As all DosBox or ScummVM releases it use custom build of DosBox/ScummVM. For DosBox its usually DosBox-CS by GOG (mean -CloudSaves with just difference where temporar changing ingame folder files stored are - redirected to in game new folder cloud_saves instead of ...user/AppData/DosBox/). For TR since 2018 or 2019 (or even early?) it use 3rd party custom DosBox (again with changed CS above it) with nGlide emulator additional support (as in remember).

So yes it did use custom DosBox. There was no recent, at least not since 2019, updates. And as i remember it didnt state it is SVN (mean testing) build. Are you sure it wasnt you or some OS/3rd party client service did overwrite existed build?

To play on Linux for a DosBox/ScummVM releases here that didnt get native OS Mac/Lin builds you need just install native emulator above GOG's one. Same for Win (probably better than GOG). Do not use Steam Proton here. I mean no need to.

For reference/further info at GOG support site there is PCGW article link. Look to additional info on each game articles on PCGW (with wine/protondb and else links there), DosBox article (seems much informatics for me no need go to DosBox wiki linked there), GOG article and Linux article.
Go to each game GOGDB org website pages to see when last update did fome and which build GOG use (there is GOG Galaxy filelists where DosBox-CS2 and nGlide files listed).
http://www.gogdb.org/products?search=tomb+raider
(there is also PCGW articles linked on game pages on GOGDB same way as PCGW link to GOGDB on each page).

Okay, my bad. I am confused by GOGDB then, because it says DosBox = no with TR1.

But you guys are both right, the 1+2+3 pack shows yes, as the store page entry, too.

But to add further to the confusion and in my defense - I also used a source port via Lutris. So mine isn't using DosBox. There aren't even any GOG files left in the install directory.

If you are using Galaxy, where it says Installed, click that, click the game's portrait, click the two little lines with circles on them in the upper right, click Manage Installation, click Configure, and from there you can select which version of the game you want to roll back to.

As for finding old install files? Once the offline downloader page gets updated, I don't know if that is still possible, but you can roll back to older versions provided it doesn't get updated too much like say Baldur's Gate 3.

The Steam Deck is a PC. It used to work fine. Kindly go away I don't really know what you mean by reason. I have literally not clue how to do this, and was not even aware that this is possible. Is that not a reason?