The leadership can block anything they want on a work-supplied device. They might get the Works Councils on board but generally they can.

However they can not reduce private social life to zero at work [1]. And there the employees are guaranteed privacy. NB: This was pre GDPR.

This however does not go so far, that people must be allowed private chatting apps on work phones. They can and should use their private phones.

[1]https://www.jacksonboyd.co.uk/barbulescu-v-romania-work-rela...