Are EU citizens in the UK the world’s only undocumented legal migrants?

Probably.

In case you didn’t know that… indeed, legally residing EU citizens in the UK have NO PROOF of that so there is never a guarantee that when they leave the country for a holiday or a work visit abroad, they will be let in again.

Why? If you need to ask that, you’re asleep.

But why did the EU go along with this???

https://www.theguardian.com/uk-news/2021/jan/16/nhs-worker-stuck-in-kenya-as-settled-status-documents-are-rejected