I can give you the example of France, my country where they are illegal, this is a relevant legislative text: https://www.cnil.fr/fr/la-loi-informatique-et-libertes#artic...

Translated with google: "It is prohibited to process personal data which reveal alleged racial or ethnic origin, political opinions, religious or philosophical beliefs or the trade union membership of a natural person or to process genetic data, personal data biometric for the purpose of uniquely identifying a natural person, health data or data concerning the sexual life or sexual orientation of a natural person."

It comes with the qualifier.

> for the purpose of uniquely identifying a natural person

So making a poll in your company to identify how many are in a trade union is not illegal by that article while building a database that contains the fact that John Doe is in a trade union would violate that law.