> Remove it when the domain is no longer for sale. The convention has no “not for sale” value; absence is the only way to say no.

Since most domains that are for sale don't currently have such a record, it would be wrong to assume they are not for sale. Therefore, absence of such a record does not explicitly mean "not for sale." It's no different from a house: a "for sale" sign in the front yard means it's for sale, but lack of a sign doesn't mean it's not for sale; it could mean the owner doesn't wish to advertise that way, or at all.