For example, you have a 'right to object' to a data controller doing further processing of your personal data, and you have a 'right to be forgotten' in which case we should not keep your personal data any longer -- but we need to remember enough that when we encounter you again, we recognize that you've objected to further processing. How are we supposed to know in future interactions that you've opted out if we've deleted all mentions of you and your PII?
What does 'deletion' mean? If I have a DB which is based on an append-only WAL which can include write and delete "operations", which mean the DB will respond to queries as if the record were deleted, but the record is definitely on disk, and still gets read into memory but just isn't returned as part of any query, but someone with access to the machine could still in principle read it ... is it 'deleted'? Are you 'forgotten'?
What if you're gone from the DB but an old DB backup in cold storage still has a record? What if a columnar file for the datalake in block storage still has a record but you're gone from all DBs that are part of online systems? What if no DB has your raw PII, but your IP was added to a bloomfilter or other sketch datastructure, so it can't be read back out but we could potentially identify with some confidence that your IP had previously been in our logs?
I totally think GDPR was a step in the right direction, and I wish my own country would adopt a strong data privacy law. But I also wish that the EU had set up e.g. a certification system, a large set of reference examples for how pieces fit together, or something to give implementing parties some confidence about whether they're doing it correctly.