There are 2 cardboard panels up under there...1 - #459-195 - is the mud shield & is definitely needed on later cars where the radiator is in the forward position ('77-'80)...its available new; the other #475-255 - is the one you're talking about - its not available new...all it really does is protect the bottom of the radiator a little & duct air coming in through the hole in the front valance....were I to guess, I'd guess that 95% of MG's on the road today don't have one (my '74-1/2 GT is the only car I own with one! & its supposed to be on al MG's from '62 until '76)
If you find a used one in a salvage yard or auto jumble - buy it!
But don't worry about not having it.