Important
Using Azure ACS (Access Control Services) for SharePoint Online has been retired as of November 27th 2023, checkout the full retirement announcement to learn more. Using Azure ACS outside of the context of SharePoint was already retired on November 7th, 2018 and is end-of-life now.
Retirement means that the feature will not get any new investments, but it's still supported. End-of-life means that the feature will be discontinued and is no longer available for use.