Most refundable fares don't have any restriction other than you have to request the refund prior to the expiration of the ticket validity (generally 1 year from date of purchase, but I think currently through 12/31/22 on many/most tickets).
But the details would be spelled out in the "Penalties" section of the detailed fare rules (which one should always save/PDF when booking as it's hard to get them after the fact). There would be language like "TICKET IS NON-REFUNDABLE AND HAS NO REMAINING VALUE IN CASE OF NO-SHOW".

But actually looking through a good handful of fare rules for past trips (pre- and post-COVID), this language (or similar language) does not actually exist even in many of my nonrefundable tickets, but rather seems to be something added relatively more recently (possibly some time between Feb-21 and Aug-21). There also doesn't seem to be anything in the Conditions of Carriage (at least domestic) either that would over-ride the lack of this language in older tickets. In any event, regardless of refundable/non-refundable, reading the fare rules (specifically "Penalties") is the key.