Yes, STARFIELD will have mods on XBOX. No reason not to, since SKYRIM and FALLOUT 4 both do and remain active games a decade after launch and more.
The real $64M question is "Will the *mod creator* run on XBOX?" Say in developer mode. With STARFIELD a play anywhere game XBOX-focused gamers modders won't have to buy a second copy to start modding but we'll still need a STARFIELD-capable PC to create.
Alternately, MS might offer up a custom version of Windows 365 with the Creator (charging, of course) for modders without a gaming PC. It would open up modding to a larger community without requiring an XBOX version of the creator.
My interest is I want to take up asteroid mining by without having to blow them up and "dock" with them. I suck at docking. (sigh)