The other answer is correct that recovery codes should be hashed. The NIST published guidance on this in 800-63b section 5.1.2, Look-Up Secrets, stating as much:
... A common application of look-up secrets is the use of "recovery keys" stored by the subscriber for use in the event another authenticator is lost or malfunctions. ...
Verifiers SHALL store look-up secrets in a form that is resistant to offline attacks. Look-up secrets having at least 112 bits of entropy SHALL be hashed with an approved one-way function as described in Section 22.214.171.124. Look-up secrets with fewer than 112 bits of entropy SHALL be salted and hashed using a suitable one-way key derivation function, also described in Section 126.96.36.199. The salt value SHALL be at least 32 in bits in length and arbitrarily chosen so as to minimize salt value collisions among stored hashes. Both the salt value and the resulting hash SHALL be stored for each look-up secret.
For look-up secrets that have less than 64 bits of entropy, the verifier SHALL implement a rate-limiting mechanism that effectively limits the number of failed authentication attempts that can be made on the subscriber’s account as described in Section 5.2.2.
Despite this, some prominent sites (including GitHub, Google and Facebook) store them in plaintext or reversibly encrypted, allowing users to view them at any time.