For clarity: European Convention (and Court) of Human Rights law, not EU.
There's post-Brexit significance since the UK continues to be a signatory to the ECHR even though they've now left the EU. Being an ECHR signatory is a requirement of being in the EU, not vice versa. (Countries like Turkey and Russia are also ECHR signatories.)
It's actually European law that's binding here, and extradition / deportation denials based on human rights concerns are nothing new - there have been bans on deportations even to EU members such as Greece (https://www.gesetze-bayern.de/Content/Document/Y-300-Z-BECKR...) or Hungary (https://www.keienborg.de/2018/02/19/die-12-kammer-des-vg-due...).