Valve has recently published the Steam hardware survey results for March 2021. Linux took a market share of 0.85% for the month, which is very similar to where it was in March 2020. According to GamingOnLinux, the overall trend is still positive. As you can see, however, Linux's market share on Steam has been oscillating quite a bit as of late.
Ever since Steam Proton launched back in August 2018, Linux market share has steadily risen. In addition, the fact that Linux has maintained a consistent percentage as the Steam userbase grows also means that the raw number of Linux users is rising. Unfortunately, the growth is not enough to put it over the 1% milestone and is a far cry from its ~2% market share back when the native Linux Steam client launched.
Easy Anti-Cheat and Multiplayer Games
The biggest issue with playing Windows-only games on Linux is Easy Anti-Cheat or EAC. Many popular multiplayer-focused games utilize EAC. Unfortunately, EAC does not play well with Proton or Wine. On ProtonDB, huge multiplayer games like PUBG, Apex Legends, Destiny 2, and Rainbow Six Siege are considered "borked" or unplayable thanks to EAC.
Why does EAC games not work on Linux? It's due to the fact that the software issues system calls (or syscalls) to the kernel level. Wine and Proton, however, only addresses the API level. So when a game like Destiny 2 sends a syscall to the Linux kernel, the system basically goes "WTF is this?" and the game crashes (or simply not boot).
Games that use EAC issue syscalls to the kernel level. Wine and Proton only translates at the API level hence why EAC games crash. Source: Brett Gardner
The vast majority of games I play are singleplayer and the few multiplayer games that I do own have a native Linux version. That said, while the EAC issue does not affect me, it does affect a large number of gamers who want to game on Linux, but cannot play the aforementioned games above. In my opinion, unless Codeweavers and the Linux kernel developers can figure out a way to have EAC play nice, then Linux's marketshare will not hit the 1% milestone.
Wayland and X.org
X.org has been around for several years and the majority of distros (and the desktop environments they ship with) utilize X11. The problem is that (1) it is very old and (2) its code is incredibly bloated. You can read the more technical issues with X.org on this article published back in 2013 (X.org hasn't really changed that much since then). From a gaming perspective, you can't use HDR under X.org which may be a turn off for some gamers who want to have the best visual experience.
The Wayland protocol aims to fix the issues that X.org has. Here's a simplified explanation from The Linux Experiment on the differences of how X.org and Wayland work:
Sounds good, right? Well, the problem is that the Wayland protocol has been around for quite a while and it's still not widely adopted. That's because when switching from X.org to Wayland, you have to re-implement a lot of basic features. When it comes to gaming, Wine and Proton expect an X-server to work (8:36). XWayland was introduced as a middle-ground to this, but it does not work perfectly. To be fair, though, average gaming performance between X.org and XWayland are about identical.
The Silver Lining
Last month, kernel 5.11 launched and it introduced a new function called the Syscall User Dispatch that may be the beginning of addressing the EAC issue. To put it simply, Syscall User Dispatch redirects the syscalls to Wine or Proton, preventing the Linux kernel from flipping out in confusion.
This does not mean that all of a sudden, all EAC games will work just like that. Syscall User Dispatch is still young, but at least the first steps have been taken.
Wayland is making some progress lately, too. KDE Plasma 5.21, for instance, launched in February with better Wayland support. The Ubuntu 21.04 beta just released and it features standalone XWayland packages and better Wayland support, as well. OBS Studio has recently implemented Wayland capture support which is big news in the content creation department. That said, I don't expect Wayland to be mainstream anytime soon.
Lastly, Proton continues to be updated. Just yesterday, Proton 6.3-1 launched, adding more playable games to the list. VKD3D-proton, the DirectX12-to-Vulkan translation layer, is making progress at making DX12 games more playable on Linux.