Re: industry practice?
Standard industry practise on running t'internet has tended to be to leave things as they are in as much as that's possible when something is later changed.
And if that isn't possible or desireable, then to have long transition periods - and try to minimise disruption.
Clearly nobody had any expectation of a country leaving the EU, so there aren't any policies in place to deal with it in almost any area. But that left the .eu domain with a few choices. They could:
a) Just let exisiting domain holders keep their domains - but not allow new ones to register.
b) Turn a blind eye to the whole issue. Did they really do any checks in the past? Certiainly when my company have been offered .eu domains by registrars there haven't been any extra checks mentioned over-and-above "can you pay".
c) Have a year or so of transition period - or force complying with the rules when the domain expires.
d) Put in place some kind of hybrid a and c - given that they're potentially losing 10% of their registered domains in one go.
d) Allow existing domain owners to pay a one-off fee to block future use of that domain - but to lose control of it themselves. As the dot.xxx registrar lets you do.
e) Take the extreme approach and nuke all domains from day 1 of Brexit.
That last is pretty unusual. It's not as if the .eu domain is like the proposed .bank one - where they were going to make proper checks that domain owners were real, regulated banks. I doubt any effort has been put into making sure nasty foreigners wreren't registering .eu domains before all this kerfuffle.
Which makes this look like a deliberate political choice in order to cause as much disruption as possible. The fact that it's in such a minor area - and there are unlikely to be many companies who do anything other than re-direct their dot.eu to their main site - just makes it look more petty and pathetic. But seemingly that's the policy path the Commission has tried to pursue, when possible. They're not sticking to internet governance norms, but it's not in an important area and the Commision have been delegated the authority to control this domain by ICANN - so they can't really complain about them using it as they see fit.