Valve has today announced Counter-Strike 2, which will be arriving in Summer 2023 and they've released some details on what's changed.
This will be a big free Source 2 upgrade for Global Offensive, with some of the features being announced now including responsive smoke, sub-tick updates for more responsive gameplay, overhauled maps, Source 2 tooling for the community, your whole inventory carries over from CS:GO, higher resolution models for basically everything, improved visual effects, an upgraded UI and much much more.
They're doing a Limited Test too, which is based upon invitations. Seems like they have more to announce, as the limited test and the features being revealed are "a subset of Counter-Strike 2's features, so that major issues can be resolved before the summer" and they will reveal more in "a few months".
As for how to get into the limited test? They will be selecting people based on recent playtime, trust factor, Steam account standing and so on. If you are chosen, you get a notification inside CS:GO, so you'll need to keep checking back with more players being added over time.
The limited test is Windows-only but SteamDB shows it all clearly set up for Linux and macOS too. It's likely just to get the main technical testing done they're keeping the platforms smaller to get it finished.