Remapping of blocks isn't the problem here. IANAL, but you should be encrypting all the data for GDPR compliance to begin with. With it encrypted, how the storage device chooses to map its blocks is completely irrelevant. When the key goes away, all the data is erased
by definition. Throwing away the key is one of the easiest ways to comply with Right to be Forgotten, from what I've seen.
"Oh, but the key is still on the hard drive!" you would likely complain. No... the key could be stored any number of places that won't leave a copy. If you store the key on a TPM/HSM, then tell the TPM to delete the key, short of finding out that the TPM manufacturer failed to perform their duty in erasing the key, and simultaneously discovering a 0-day exploit to go in and retrieve that deleted key, it is gone. Full stop. It is unreasonable to ask everyone to do something impossible like prove that their TPMs are working properly, when even the TPM manufacturers cannot definitively prove this, so that is clearly not what the law is asking. You might as well ask companies to prove that they've never been hacked. How can anyone prove that?
Conceptually, one of the simplest ways to comply with this would be to perform database failover once a month, since you have a month to comply with these requests. Before the failover, you would ensure that all data is deleted that has been requested to be deleted. Then you bring up a new database replica, stream the remaining data to it, failover, and then destroy the key stored in the TPM on the primary and any other replicas. All that deleted data is now gone forever.
There are more complicated architectures where you could have per-user keys which are stored on an encrypted volume, and then you would be managing a handful of keys in the TPM to handle rotation of the encrypted volumes. Once a month, you copy the per-user keys that haven't been deleted onto a new encrypted volume, then tell the TPM to delete the key for the old encrypted volume: garbage collecting the deleted keys in an unrecoverable fashion. This avoids the hassles with having to failover a database so frequently, although having that level of automation around database failover has advantages too.
I'm sure some companies are content with treating the deleted data as erased, as long as their drives are properly encrypted. A rogue employee could potentially recover the data while the hard drives and the keys are still together, but the hard drives will be worthless once they're separated from the keys after they fail and are thrown away. A rogue employee could have stolen the data before it was deleted anyways, so how do you prove that rogue employees don't exist? Is this level of erasure good enough? This one I'm a little iffy on, but it still completely resolves your issues with the block remapping.
GDPR makes reference to "taking account of available technology and the cost of implementation", so I think it is fair to say to that GDPR is not asking you to erase any butterfly-effect waves that have propagated out from the read/write head of your hard drive. If the data is not recoverable by any known means, that is the purpose of the law, isn't it?
I will repeat that I'm not a lawyer, but your argument feels like a strawman designed to paint the Right to be Forgotten as something that no one can comply with. Your comments come off as snarky and acting like you think you're so clever. As if no one has ever thought about these problems before.