As you know, SharePoint Online has been providing workflow capabilities since its beginning.
Well, after having decommissioned SharePoint 2010 workflow in 2020 (see https://t.co/VOoO4cow3w), Microsoft is retiring SharePoint 2013 workflow on SharePoint Online.
SharePoint 2010 workflow and SharePoint 2013 workflow remain supported on-premises until 2026 for SharePoint 2016 and SharePoint 2019.
As announced in 2020, new tenant have been provisioned with SharePoint 2010 workflow and SharePoint 2013 workflow turned off.
If you have been using SharePoint 2013 workflow, you can now disable the creation of new SharePoint 2013 workflow using the below PowerShell command (available in SharePoint Online PowerShell version 16.0.21513.12000 – available for download here https://www.microsoft.com/en-us/download/details.aspx?id=35588).
Connect-SPOService -Url https://<your tenant>-admin.sharepoint.com
Set-SPOTenant -StopNew2013Workflows $true
Existing SharePoint 2013 workflow will continue to work but you should plan to move to PowerAutomate and/or PowerApps.