Soithwest has open seating which is accessible through a ticket. It does not have seat reservations or assigned seating, or at least it hasn’t in the past but will apparently will start having seat reservations in the near future.

I know phrasing has stupid nuances in different contexts but there are differences between a reserved seat, reserved seating, reserving a table, etc. and while table reservations often mean first available seat reservations generally include assigned seats in my and the OPs experience.