Thanks, I have. The first result on Google[0] says that pseudonymized data is still considered personal data.

[0] https://dataprivacymanager.net/pseudonymization-according-to...

Yes, pseudonymization is reversible because you keep the PII separate from the rest of the data associated with the user.

The reason you pseudonymize in the first place is that you can delete the PII part, after which the data becomes anonymized, which you can keep.

That's incorrect. If the psuedonymized data is enough to fingerprint a specific individual (maybe with some additional data), the GDPR says you still can't keep it.

Correct! Which is why you need to pseudonymize correctly.

Good luck truly anonymizing the activity feed of any user on any social network. Good luck anonymizing support tickets that include stuff like "I'm running this version, on this device, and trying to do that". Good luck anonymizing messages between users that includes nicknames.

Blockchain is such a terrible idea because even if today you're thinking "Well, I guess storing X is fine, it can't identify a user", tomorrow you might learn that it can. And there will be nothing you could do about it. And we haven't even started talking about bugs, which all software has (including DAOs and smart contracts...), that might cause you to store stuff on the chain, publicly and irreversibly. The future is going to be fun!