Note I said delete or anonymise. The point is that the information shouldn't be personally identifiable once the person has asked to be forgotten (within reason of course - you still have to hold as much info as the law mandates whether that person likes it or not).
You're welcome to keep the anonymised data and do whatever ML or other analytics with it.
> If you do not wish someone to have your data, don't sell it to them.
There are plenty of services that gather far more information about you against your will than ordinary folk imagine, IIRC Google and Facebook's tracking of your activity via their ad networks even when you're not visiting pages on their domain. This isn't acceptable, and lots of ordinary people aren't savvy enough to use adblockers or script blockers, yet they never consented to that type of data collection.
The GDPR at least gives them some say in what happens to the data that was collected about them.