Yes, in Germany it is illegal. AFAIK, the law states something along the lines of having "proper shoework" in order to operate the vehicle.
EDIT: Whoops, no, it's actually legal [1][2] (in fact, it's perfectly legal to drive a car naked in Germany in public - though you shouldn't leave your car then, because being naked in public is not actually legal, the fine-print is that your car's interior is considered your private space), but you may get into trouble with insurance. While searching, I think I may have read that it actually is illegal in Switzerland, but don't take my word for it...
What a fantastic legal quagmire, thanks for looking that up
Tangentially, I once had German autobahn police laugh at my 'gummistiefel' when they pulled me over... The opposite thing of driving barefooted, but hardly proper shoework for driving a car
Barefoot is quite common, particularly amongst those that wear high heels when out of the vehicle.
So long as you can push the pedals safely all is fine. The Highway Code does have a few words about wearing appropriate footwear, the biggest thing is insurance though. If you are driving in high heels and it all goes wrong then your insurance isn't going to necessarily help.
