Yes and no. PII needs to be removed. The rest of the data needs to be anonymized. Right?

But an email is PII so clearly this would breach GDPR.

Keep a hash instead of the original email address?

Whatever for? Just delete it. Keep the account tombstoned so its name can't be reused. Keep what content you can and want. Delete the PII and any metadata you're contractually and/or legally required to.

Did a glance through that thread and it didn't seem like there was a strong consensus on how to respect GPDR while maintaining historical data for reporting purposes. Any best practices?

Having worked in a HIPAA regulated space, I can say that hashing the username, such as the email address, for login purposes can allow for account recovery if the credentials are retained. At the same time the cleartext username and other PII can be stored in an object that is both encrypted at rest for its lifetime, and on top of that has its sensitive fields overwritten upon logical deletion. Account recovery cannot recover non-credential derived PII but that is a small annoyance to the user in order to be compliant and trustworthy. The internal user ID should be used throughout downstream reporting rather than actual PII for the sake of continuity and privacy.

Yes this is all quite do-able. It is often much easier to implement from the beginning, than after the fact.