A quick search showed that it is illegal in Spain, but not Germany, France, or the UK, and nothing really showing up either way about any other European country. Though I didn't dig down enough to find legal advice I would actually trust.
For private drivers it is not illegal per se. But if you have an accident, they (police, prosecution, insurance) will quickly blame it on unfit shoewear.
There is (legally accepted) consensus in Germany and Austria that a regular, closed shoe without heels gives you best control.
Barefoot is IMHO treated unfairly, if you're used to drive barefoot. And living in a country where people wear shoes most of the time, they will assume you're not used to it.
