Indeed, guys, I also feel that dropping support is one thing while cutting the access to your whole library is totally another.
And yeah, I've been trying to find a way to at least keep all the games that I've already owned working, but I have no clue as to how to back up an archived version of the client. My plan was just to download the "most important" games, and then when 2019 comes, cut the internet before opening the client and then switch it to offline mode. It's probably too stupid, but I knew no better way by my own.
Backing up the Steam client is pretty easy, because it's actually completely portable: You literally need to just zip the entire installation folder. Yeah, that's honestly it. Any games that you want to keep that don't have extra DRM (Denuvo, Uplay, etc.) will all work as long as they've been downloaded and run at least one time each; if Steam shows that the games are installed, then you're golden. You can even copy/paste it to another machine with the same OS, and it'll work.
If you don't want to put in your email and password every time you start it, put it into Offline Mode before you back it up, and also back up HKEY_CURRENT_USER\SOFTWARE\Valve\Steam\AutoLoginUser and HKEY_CURRENT_USER\SOFTWARE\Valve\Steam\RememberPassword from your Windows registry; run a Steam client backup, cancel out at the login screen, make sure it's totally gone in Task Manager, and add the registry keys back to re-enable Offline Mode, and no password will be necessary anymore (all of your login info is stored locally in the Steam installation folder that you will have backed up).