It's the registrar rules being enforced exactly like they always have been. The Rules say "EU only". If the UK is no longer in the EU then the simply conclusion is that the domain ownership will be revoked.
The rules say: EU, or Iceland, or Norway, or Liechtenstein.
also, the rules are not set in stone
Until the rules are changed, UK domain owners will be thrown out in March 2019.
No; UK domain owners will be prevented from renewing their domains, but it doesn't say they'll automatically lose them. From what I can tell, if you renew just before March 2019, you can keep your domain until 2029. Although, the Registry will be able to revoke them, so it's not guaranteed.