Nice list, but i have Extra Engines i want to include:(Only engines considered "legal",note: some of these i didnt test myself,but this may contain reverse engineering projects . )
For Omni-Engine (Lego Island branch only)
comments: it also adds new features and improvements to Lego Island like multiplayer(Experimental),WASD Support,etc.
ISLE-Portable:
Play it on your browser
Source code
For Touhou's/Seihou's engine games
PyTouhou (For EOSD branch only):
Caveats: missing some features,git server down due to LLMS,pre-build binaries are quite old.
Downloads
Source Code (likely down)
for PC-98 branch only:
Caveats: it doesn't seem like it has cross-platform support.
comments: seems like its mainly focused on reverse engineering
Source code
marginally releated to Touhou but not really part of Touhou Project:
For Seihou Project (Shuusou Gyoku branch):
comments: Windows only binaries,Linux binary available as a Flatpak.
Source code
For Source engine games
comment: steam_legacy branch only,also does not work with TF2 Branch games,despite its name it also works on Windows,Macos and Most Linux distros.
Wiki about it
The SDK is also a thing too,but am not gonna include it here.
For Id tech 4 engine games(i think DOOM 3 branch only)
dhewm3:
Source code
RBDoom3BFG(For BFG branch):
Source code
for NetImmerse engine games(The Elder Scrolls III: Morrowind branch only)
openMW:
Download
Source code
I mainly includes engines i have heard off
yw,you may include them on the list if desired.