https://learn.microsoft.com/en-us/sharepoint/dev/solution-guidance/security-apponly-azureacs
Azure Access Control (ACS), a service of Azure Active Directory (Azure AD), has been retired on November 7, 2018. This retirement does not impact the SharePoint Add-in model, which uses the https://accounts.accesscontrol.windows.net hostname (which is not impacted by this retirement). For more information, see Impact of Azure Access Control retirement for SharePoint Add-ins. For new tenants, apps using an ACS app-only access token is disabled by default. We recommend using the Azure AD app-only model which is modern and more secure. But you can change the behavior by running ‘set-spotenant -DisableCustomAppAuthentication $false' (needs the latest SharePoint admin PowerShell).