[CookWithMe]

> The main concern with this approach is that the event store is no longer immutable

I think what happens when you delete all events of a (deleted) aggregate root (such as a customer who requested to be forgotten) can be interpreted more charitable, in a way that the event store can still be called immutable.

If you look at a functional programming language, you can not force a data structure to be removed from memory. However, that obviously doesn't mean that your program consumes infinite amount of memory. If a data structure is not referenced anymore, it'll be removed from memory (by the GC). Your program itself didn't mutate the state of the data structure, so from that point of view everything is still mutable.

Now, let's apply the same principle to an event store: A deleted aggregate root (the to-be-forgotten customer) should have been removed from all projections (as required per GDPR). If you replay the events, it shouldn't matter to the final state of a read projection whether it processed the events belonging to this aggregate root, or not.

Therefore, one could interpret that removing the events of a deleted aggregate root in a GC-like fashion leaves the event store immutable, in the sense that my program(s) can't mutate the state (themselves), and their output doesn't change.