This timeline is not acceptable for the real world. I have often said that Microsoft lives in it's own world. WIth everything going on in the business world right now, how do they expect enterprises to get all workflows re-done in less than 4 months. We are in the middle of migrating from SP2013 to SPO to get off old servers and technology. Due to time and resource constraints we made the decision to migrate workflows as-is then spend 2021 rewriting them using PowerAutomate. I understand removing old technology, but you HAVE TO give your customers more time to get things in line. This timeline needs to be extended and give at least a year warning. I put in a service ticket about this as someone suggested, they just called me and said "nothing we can do". I'm also going to escalate this to our account rep. We have to make sure Microsoft hears from people on this one to extend it.
Also, vote for the extension here: https://sharepoint.uservoice.com/forums/330318-sharepoint-administration/suggestions/40856713-postope-designer-2010-workflow-engine-retirement