What is kept, and for how long
Deletion happens automatically once the period is over. It can take up to two days after that for an item to be gone everywhere.
The suppression list keeps no addresses
An address on the suppression list must never be written to again, so the list is never cleared. To keep it without keeping anyone’s address, each entry is stored only as a one-way hash of the address (lowercased, with any+tag removed). A pitch to that address, or to any alias of it, is still refused, because the same hash comes out.
GET /v1/suppressions lists each entry’s email_hash, its reason and its date. To check whether an address is on your list, hash it the same way: the hex SHA-256 of the address, lowercased, trimmed, and without a +tag.