Are there any developed countries that practice economic libertarianism? ie, no social social safety net, no minimum wage, no healthcare, no retirement benefits, no spending beyond military and some infrastructure?
No. This exact subject has been asked before. Try the search function.
Has there ever been a libertarian country (deeloped or not)? The closest I can think of is medieval Iceland.
Among modern countries, perhaps Hong Kong came closest. Before that, I’d say the U.S. before Roosevelt might have been up there.
But none have ever been fully Libertarian.
The US might have been up there on economics but not in other areas. By the way you meant Teddy, right?
Hong Kong might be a good example, but I don’t know about any areas other their economy.
Of course Hong Kong isn’t a country, though. 
Except that HK has quite a good social safety net and healthcare system. I think some of the former USSR countries are moving very economically libertarian.
Hong Kong doesn’t have military expenditure either.