There are two general types of OpenStack charms: one that does use channels and one that does not (legacy).
With the channels type, a channel is dedicated to a single OpenStack release (release N-1 will be technically supported to assist with upgrades). This means that a charm that works for a recent series-openstack combination will generally not work on an older combination. Furthermore, there is a need to switch to a different channel in order to upgrade to a new OpenStack version - but not to a new series.
The Charm delivery page explains how channel charms are distributed to the end user.
For the legacy charms, unless stated otherwise, each new revision of a charm includes all the functionality of the previous revision. This means that a charm that works for a recent series-openstack combination will also work on an older combination.
The development of legacy charms has stopped at the 21.10 release of OpenStack
Charms (and at the 21.06 release of Trilio Charms). The last supported
series-openstack combination is