DepFreeze (DF) generally happens at the same time as FeatureFreeze. It ends when all projects completed their RC1 publication and have a proposed/* release branch.
Once DF kicks in, you are no longer allowed to accept proposed changes containing new requirements to the openstack/requirements repository. Such proposed changes should be temporarily rejected by the review team and postponed until the next series development opens (which should happen when all RC1s have been published). Upgrades to existing Oslo libraries and OpenStack python client libraries are generally considered alright.
DF facilitates the work of downstream OpenStack packagers by not gratuitously adding dependencies toward the end of the cycle, when they don't have as much time and freedom to add new dependencies to their distribution. It helps ensuring the timely packaging of OpenStack releases in distributions.
If you want to propose requirements changes during DF (that is, if you believe has an acceptable benefit/disruption ratio), you should first raise an openstack-dev ML thread with [depfreeze] in the subject line (in addition to the affected project), discussing the proposed change and its merits. If the thread validates the approach, you can propose a change, referencing the discussion.
The Release manager, with the assistance of the core developers of the associated product, will evaluate the request and grant or deny the exception. The farther we are in the release cycle, the less likely it is for the exception to be granted. Remember that the next cycle is just a month away :)