Passport confiscation is a common sign of modern slavery.

    "he was lured in with the false promise of a well-paid job in the UK"
    "The gang confiscated the passports of all their victims"

It's not legal. There are definitions of "Modern Slavery" and descriptions of the practices and warning signs because it is still an issue in contemporary times, including in Europe.

https://www.bbc.co.uk/news/articles/c2kdg84zj4wo