RE: CRM On-Prem to D365 online migration (Mail Merge/Dialogs/Task Flows?)
See info below
Task flows are used to create a step-by-step data entry form for common tasks, such as after-meeting follow-ups.
Task flows are deprecated and will be removed by October 1, 2021. They will be replaced by the immersive experience for business process flows that is planned for release. More information: Business process flow immersive experiences
Process Dialogs are deprecated
You can use a process dialog to create an interactive step-by-step data entry form that requires user input to start and run to completion. When you start the dialog process, a wizard-like interface is presented; users make selections or enter data as they progress through each page of the wizard.
Process dialogs are deprecated and will be removed by December 1, 2020. Suggested replacement options include Business Process Flows or Canvas Apps. More information: Replace dialogs with business process flows or canvas apps
https://docs.microsoft.com/en-us/power-platform/admin/work-mail-merge-templates
The mail merge template feature has been deprecated and is not supported. In Dynamics CRM 2016 (version 8.0), we introduced server-side document generation using Word and Excel templates. You can use these templates to provide standardized documents or customized data analysis for your organization.