Ah, my mistake. I had read in the FAQ that it does not guarantee that data is stored forever, but overlooked the part about preserving checksums specifically.
To be very pedantic, there are two separate services: The module proxy (proxy.golang.org) serves cached modules and makes no guarantees about how long cache entries are kept. The sum database (sum.golang.org) serves module checksums, which are kept forever in a Merkle tree/transparency log.