As per https://matrix-org.github.io/synapse/latest/admin_api/user_admin_api.html#deactivate-account

When you deactivate an account it will clear all account data, so it will remove all 4S data (this includes the server side megolm backup encrypted key)
So even if you reactivate the backup will be unusable.

Notice that it's possible to setup a backup without using 4S (in this case it would be possible to recover your key backup).

Nonetheless, it looks sensible to delete the backup when deactivating the account.
If not, it would be good to then save the 4S account data keys and to restore them on reactivation.