I've been away for a few years, and now I see that there are traits that are in red, unable to be generated or mutated to. Apart from the current one, will those ever cycle back in, or are Natureguard etc. locked in and unobtainable going forward?

Complete guide to Restricted Traits here.

It's possible that a new theme may be released with an older restricted sets, as that did happen for the 2016 Black Friday Wheel.
Obtaining a Restricted Trait Disruptor can put a trait on certain pre-existing pets (see post 2's "Restricted Trait Disruptors" section for the details there)
The Guardian Angel Griffi (Galactic trait set) have been available the past two winter fests as activity prizes.
And Valentines Lace and Lucky Dust can be Obtained via their new RTD's, via their old genetic mutators (Post 2, section "Restricted traits found on all pets"), and generated onto new pets during their respective festivals (you can currently generate Lucky Dust on any pet, for the duration of the Fortune Festival)

Thanks! I'm guessing we don't know if the natureguard etc will ever come back?

Asside from v-lace and lucky dust there was never a way to generate a pet with the restricted trait set. Tho there is special mutators that allow you to swap one trait of any kind of pet for a restricted ( following the trait set from the mutator). Those mutators may become avalable again in the futur.