Since it has been announced that Microsoft is officially ending support for SharePoint 2010 Workflows in November 2020 and SharePoint 2013 Workflows being disabled for new tenants in SharePoint Online...What is the impact to Nintex Classic Workflows as they show up as SharePoint 2013 Workflows in SharePoint Designer?
Starting in November 2020, SharePoint 2013 workflows will be turned off by default for new tenants. Microsoft will provide a PowerShell script to let customers to activate the SharePoint 2013-based workflow engine for tenant as needed.
SharePoint 2010 and SharePoint 2013-based workflows will continue to be supported for on-premises SharePoint 2016 and SharePoint 2019 Server platforms until 2026.
Note that Nintex Workflow for Office 365 uses SharePoint 2013 workflows.
The deprecation of SharePoint 2010 workflow has no impact to Nintex Workflow for Office 365. Please see article - Nintex Workflow for Office 365 not impacted by SharePoint 2010 workflow deprecation.
With regards to SharePoint 2013 workflow