If you look into the data Steam OS Holo s listed and it is 45.3%. Arch separately is second at 7.9% and then third is the Flatpak installs across all Linux versions at 6%.

The changes are more difficult to interpret as Linux is growing overall so changes between Linux distros are difficult. For example a small decline in overall share may still represent an increase in total numbers. While Steam OS is up another 3% points, other distros combined are up more - Ubuntu and PopOS combined are up 5% points. That suggests the Linux growth is split between Steam Deck and PC users rather than purely one or the other dominating.