Thank you

Unfortunately, I discovered this help too late. I am using the client from 08/23 and would like to switch to an older, lighter client.
I have been in offline mode with the Steam client since 12/31/23 and am wondering if it is still possible to
a) do a downgrade
b) what happens if I do go online.

To be on the safe side, I run the client with RunAsDate and trick it into thinking it's 12/31/23.
RunAsDate.exe /movetime /startin "C:\Program Files (x86)\STEAM" 31\12\2023 00:00:01 "C:\Program Files (x86)\STEAM\steam.exe" -pretendeol-w10

I have blocked with "Firewall App Blocker v1.7" all EXE files of the client in the firewall and deactivated the "Go Online" option in the Steam menu via a skin.
I also set the described SkipOfflineModeWarning option in the Loginusers.vdf file to 1.

I was able to download games with STEAMCMD and STEAMCMDui and successfully integrate them into steam.
https://developer.valvesoftware.com/wiki/SteamCMDui
However, these were games that had already been installed once. The images for the library were therefore already available. I have a script to download the library images for NonSteamGames but at the moment I don't have the power to customize the script.
I would also like to test this manually first.

The library images are saved in the following folder and are saved there with the following naming structure.
...\STEAM\appcache\librarycache
AppID_icon.jpg
AppID_library_600x900.jpg
AppID_library_hero.jpg
AppID_logo.png
AppID_icon.jpg

To add the game to the client, it must be added to the Windows registry.
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Valve\Steam\Apps\APPid]
"Installed"=dword:00000001
"Updating"=dword:00000000
"Running"=dword:00000000
"Name"="App or Game Name"

I haven't tested this at the moment with new games yet, I don't know if it works with every game.
I now buy my games on CD or elsewhere, there's a lot to catch up on from the 90s.

Game keys can be activated via the following link.
https://steamhost.cn/account/registerkey

I hope my experience can be useful to others here.