yes, mods work offline.

I am already subscribed to the "no reloading" workshop mod for Serious Sam 4 Planet Badass, but I didn't see it working at all last time I played the moddable version of Serious Sam 4 Planet Badass in offline mode (I hate offline mode so much, but I play in offline mode only when I don't have internet access obviously).

Also the list of packages (all subscribed mods) was also empty in the EXTRAS screen.

The "no reloading" workshop mod for Serious Sam 4 Planet Badass does work for me when I play the moddable version of Serious Sam 4 Planet Badass in online mode (when I have internet access of course).

It seems that the "no reloading" workshop mod for Serious Sam 4 Planet Badass is working only in online mode and not at all in offline mode at least for me anyway and I don't know why at all.

I can confirm that mods definitely don't work offline. I've been trying to figure out how to make them based on guides for other games but thusfar have been unsuccessful.