It is indeed illegal, but unfortunately it still happens.

tuc.org.uk/…/workers-uk-put-more-ps35-billion-wor…

Workers in the UK put in more than £35 billion worth of unpaid overtime last year

www.cgtbs.info/index.php?option=com_content&view=…

58% of overtime hours in Spain during the 2nd semester of 2015 were not renumerated