Your contribution to the welfare state (absolutely MASSIVE public sector) is considered public information, to ensure that everyone trusts that everyone contributes equally. Not joking.
It's not considered private information, I guess. There's also exceptions to gdpr for laws and regulation, so I assume they're pretty well covered, being /the government/ and all. This data is published by the tax authority.
And it isn't something new, it's been like this since post-WW2.