Nice topic & nice to see a member looking
for explanations based on numbers. My half cent:

Syn-Spigot DB could provide some extra numbers to consider,

From
Game Releases by Year --> Instant search for 202x

2023 (Type = Game && DLC) 5,334
Windows 5132
macOS 955
Linux 588
macOS (Apple Silicon) 244
Steam Deck Playable 164
Steam Deck Verified 109
macOS (Notarized) 106
Steam Deck Unsupported 50

2022 (Type = Game && DLC) 19,185
Windows 18423
macOS 4022
Linux 2214
Steam Deck Playable 890
macOS (Apple Silicon) 749
Steam Deck Verified 547
macOS (Notarized) 458
Steam Deck Unsupported 295

2021 (Type = Game && DLC) 18,824
Windows 16591
macOS 3168
Linux 1795
Steam Deck Playable 784
Steam Deck Verified 547
macOS (Notarized) 415
Steam Deck Unsupported 356
macOS (Apple Silicon) 195

Please notice:
-I dont vouch those numbers quickly gathered
-I didnt even check if they make the most basic sense
like Windows < Yearly numbers = NonWindows vgames exist in such magnitude?!
-If you need links or help to replicate those numbers,
please PM due some paranoid arbitrary censorship we know

Of course we would also need the statistics from Steam at least, to see whether it is a GOG issue or developer issue, and even then other factors come into play.

I've certainly noticed less Linux versions for a few months now with my purchases. But it is always hard to be subjective, as I am not really buying new games, even though they might be new to GOG. There has been a reduction in Linux demos too at GOG for upcoming games.

And to be perfectly honest, GOG seem to address things in waves. So maybe we have a Linux and or MAC wave to come that will address numbers. GOG seem to have regular pet things they focus on for a while.

Did you know that Interplay and Ubisoft were the first publishers to work with GOG?

Just for fun - let's have a look at their games:

Interplay:
55 games in total, thereof
55 for Windows,
27 with MacOS support and
20 with Linux support.

That's 100% Windows support against 49,09% MacOS support, and 36,36% Linux support.

Ubisoft:
57 games in total, thereof
57 for Windows,
0 with MacOS support, and
0 with Linux support.

That's 100% Windows support against 0% MacOS and Linux support.

The numbers that are lacking now due to the inparity between the OS's, won't ever made up for again.

(mind you: we're talking native support here - I'm sure, some of the officially not supported games can be run through Wine, etc)