It took some digging to find my save games. Here's what I found for anyone else trying to backup or manually transfer save game files. If you find them in a different place, please comment here to pay it forward for future seekers.

Windows 10
C:\Users\[USERNAME]\OneDrive\Documents\My Games\Into The Breach\

OS X High Sierra
~/Library/Application Support/IntoTheBreach/

Note that both Windows and OSX have similar paths that might trip you up. On Windows I needed the OneDrive\Documents folder, not the local one (which is empty). On OSX it's the user Library (Home/Library/...) not the system Library on the root of the system drive. You can enter ~/Library into spotlight search to get to the correct place.

FYI - I was transferring the save games from a direct install in OSX to a GOG Galaxy install in Windows. I started by installing the game, without opening it. I zipped and emailed myself the entire directory, including a .lua file and my user profile folder. It was tiny. I extracted it to the windows directory, deleted the extraneous files you always get going from OSX to Windows, then ran the game. The game launched fine and seems to have all of my unlocks and achievements. I didn't have a game in progress, and still don't. ITB did start pinging every second or two, listing off my achievements. That was annoying, but I quit and relaunched and it didn't resume the list. Aside from finding the files, it was easy.

Obviously, you have some specific settings to backup your documents to the Microsoft's OneDrive.
Majority of users without those settings will have their saves in Windows 7/8/8.1/10 at the following path:
%USERPROFILE%\Documents\My Games\Into The Breach\
or like this
%HOMEDRIVE%%HOMEPATH%\Documents\My Games\Into The Breach\

Usually those variable combinations will be evaluated to following path:
C:\Users\[USERNAME]\Documents\My Games\Into The Breach\
but not always.
Actual value may vary from system to system, so the safe way is to rely on variables and not some universal absolute path.

Fair enough, though I didn't set it up. Being a Microsoft product, they installed One Drive automatically during Windows 10 Setup without asking permission.

I have the Windows savegame files and want to transfer them to my SteamDeck. I'm playing Into the Breach via Lutris and now, I want to import my savegame to SteamDeck. But where can I do this? I can't find the savegame location... Do you have clue and is it possible to use the Windows save files?