I'm guessing it to be likely the information isn't anonymised, so they can identity individual players in the data, since anonymised metrics tracking that cannot be used to identify individual players is still perfectly fine under GDPR without needing declare "we're tracking everything you do and keeping it forever and ever and ever" (and so is basic logging to keep a system running and track errors and whatnot).
Even then, pretty sure turning the tracking off for pre-GDPR versions at the server-side would be enough (just have the server-side pretend it's still working if that'd break the game somehow, but not actually record anything), since they're most likely also tracking the game version.
...So, I'm kinda being forced to conclude Paradox have chosen to inconvenience their players by saying they need to sign up to another service (incidentally means they can get even more information), instead of giving up on the tracking information for older versions? Or for some reason, the 3rd party they use for the tracking doesn't have the ability to disable based on a parameter? (In which case, I'd be reevaluating my usage of them in a consent-driven world since that seems like a useful feature).
And since the privacy policy would also need to be 'hard coded' into the release version, instead of being something pulled in over the network to allow them to update it in all versions...man, they ducked that one up.