Single Proton prefix

On Linux, Proton, whenever a Steam Windows software, a .exe or .msi file is played the 1st time, a new steamapps/compatdata/a_number folder with inside a new Proton prefix is created.

But it's possible to have Windows softwares in a single Proton Prefix, as Wine.
The key is:
LAUNCH OPTIONS: STEAM_COMPAT_DATA_PATH=proton_prefix_path %command%

I also created this feature request https://github.com/ValveSoftware/Proton/issues/8486 without success...

Optional
Conditional

Option: add proton_prefix_path/pfx/drive_c/Program Files (x86)/Steam

C:\Program Files (x86)\Steam is the default location of steamapps on Windows.

This path can be created manually or by creating the Proton prefix.

Steam
Settings
Storage
Add Drive
Let me choose another location
Add (if a partition is available to add)
proton_prefix_path/pfx/drive_c/Program Files (x86)/Steam
OK

Install Proton

The versions of Proton https://github.com/ValveSoftware/Proton/wiki/Proton-Versions (I use Proton Hotfix).

Proton:
Properties...
General
LAUNCH OPTIONS: STEAM_COMPAT_DATA_PATH=proton_prefix_path %command%
INSTALL
INSTALL TO: proton_prefix_path/pfx/drive_c/Program Files (x86)/Steam

Add a non-Steam .exe or .msi file:

Add a Game
Add a Non-Steam Game...
a .exe or .msi file (e.g. an installer like tmnationsforever_setup.exe)
The file:
Properties...
General
LAUNCH OPTIONS: STEAM_COMPAT_DATA_PATH=proton_prefix_path %command%
Compatibility
Force the use of a Specific Steam Play compatibility tool

Install a Steam Windows software:

The Steam Windows software:
Properties...
General
LAUNCH OPTIONS: STEAM_COMPAT_DATA_PATH=proton_prefix_path %command%
Compatibility
Force the use of a Specific Steam Play compatibility tool
INSTALL
INSTALL TO: proton_prefix_path/pfx/drive_c/Program Files (x86)/Steam