My guess is they will store them on tape e.g. on something like Spectra TFinity ExaScale library. I assume AWS glacier et al use this sort of thing for their deep archives.
Storing them on something that has hours to days retrieval window satisfies the court order, is cheaper, and makes me as a customer that little bit more content with it (mass data breach would take months of plundering and easily detectable).
Glacier is tape silos, but this is textual data. You don't need to save output images, just the checkpoint+hash of the generating model and the seed. Stable diffusion saves this until you manually delete the metadata, for example. So my argument is you could do this with LTO as well. Text compresses well, especially if you don't do it naively.