shop=snack is (at least for now) not used much so there is still time to migrate to more standard value
shop=snacks better follows typical shop form, as far as I can see
What you think about such change in recommendation, and moving Tag:shop=snack - OpenStreetMap Wiki to shop=snacks?
I think that with less than 150 uses of shop=snack it is still safe to make such change
Typical reason against doing something like it would be that tag in significant use is already supported, remembered by mappers, would require coding change in many projects.
This do not really apply here, and I can offer that I will correct future appearances of shop=snack to shop=snacks
See Key:shop - OpenStreetMap Wiki for how widely used values look like
See Tag:shop=snack - OpenStreetMap Wiki for documentation page that would be moved
See shop=snack | Tags | OpenStreetMap Taginfo and shop=snacks | Tags | OpenStreetMap Taginfo for taginfo
Note that no project listed itself as supporting specifically this tag value: shop=snack | Tags | OpenStreetMap Taginfo