"Closed" and "unlicensed" are not the same thing.
If an airfield has been declared "closed" by the operator or owner, then using it is trespass. If you have an emergency - who cares, land then apologise afterwards. It would be poor manners and poor practice to land under other circumstances however.
On the other hand, if it's "unlicensed", but not actually closed, then that's different. Only public transport requires a licence in place, instruction doesn't. The licence basically validates various safety procedures - minimum ATC, runway standards, fire and rescue; but those can all be in place without the licence.
We used to have a really daft rule that required light aircraft instruction to use licenced aerodromes, although microlight instruction didn't. After years of absolutely no difference in training fatalities between the two, the rule was finally removed for light aircraft training, and within a year a large number of training airfields had got rid of their licences and saved a lot of money and effort
None of which changes the universal and absolute requirement that the captain has to ensure the safety of the aircraft and the suitability of the airfield being used.
G