Anyone know how to enable tabbing out while in the game?

You have to use Quake 3 Engine Minimizer for that.

Or just use the community patch ;)

Which community patch exactly?

This is from the Mobile Infanterie Wiki: After the Quake 3 engine became open source, some people started patching Quake 3 and adding features. A certain Thilo Schulz also used this opportunity to write patches based on it for EF. A brief outline of practical changes:

EF also for Linux
Gamma bug (image too dark) fixed
Map downloads can now also be downloaded from the client via cURL (significant acceleration)
Settings are now in the user folder, not in the game folder (so each player have his own settings on the PC)
With ALT + TAB you can switch from the game to some other application and back.
Even back then already some security bugs were known and fixed.

Sadly, the direct link doesn't work, but here is the website: www.mobile-infanterie.de

This looks like it's for MP only so it's useless when you want to play the campaign. If there is a fan made patch for single player then I'm too dump to find it.

Well, Alt+Tab and the Gamma Bug is also good to have fixed in singleplayer

I found that doing Ctrl+Alt+Delete and then opening Task Manager is one possible workaround (it makes everything on screen while outside the game appear lighter/brighter though, at least for me)

The best thing that happened to my game was installing this HD overhaul:
https://www.moddb.com/mods/elite-force-graphic-overhaul-project/

Gives you the newer Quake3 version of the engine, and you can configure it properly, windowed mode, tab out and everything. Reshade included, use the Mediator.exe to disable some of the effects to your liking (like the fisheye chromatic aberration and such). Looks real good!