Pruning isn't by field type. Pruning means keeping the UTXO set and a certain count of blocks or space dedicated to storing blocks. Even pruned you are storing that chain content until it ages out of your block storage.
In order to validate you need to be able to hash the entire block. Change any field and the hash changes. The hash from last block is included in the next block so you can't tell if it is valid unless you have the entire block before that which you can't tell if it is valid unless you have the entire block before that, and so on forever back to the Genesis block.
Pruning works by validating the entire chain then discarding and trusting your own prior work.