Apparently, this article is talking about the "Legacy CS:GO Version" that was available (even after the CS2 launch) for devices that were unable to run CS2. It seems that was less than 1% of CS:GO players, so they are ending support for it, even though they claim it should still be available with reduced compatibility.
I think anyone can switch to this version in the "Beta" tab of the properties window for CS2 by selecting "csgo_legacy".
What is the legacy version of CS:GO?
The legacy version of CS:GO is a frozen build of CS:GO. It has all of the features of CS:GO except for official matchmaking.
What will happen after the end of support for the legacy version of CS:GO?
After January 1, 2024 the game will still be available, but certain functionality that relies on compatibility with the Game Coordinator (e.g., access to inventory) may degrade and/or fail.