It's note THE expected answer, but I'm using this workaround for months now: I used my "assigned to" user also in the "requestor" field. It's not so clean because you can't see weel in the request list who made the request, but it works!
Just started woking on Approval flow (because for Dynamics 365 VOC deprecation) - can't believe we cannot use an extenal non-office 365 email address already ... looking forward to the feature
With the deprecation of the SharePoint 2010 Workflow engine on the horizon this is more important than ever. We use that workflow extensively and I am unable to create anything workable because of the restrictions of my external Guest user accounts cannot resolve email addresses in the flow. This needs to at least be looked at again prior to the November 1 2020 EOL date for the SharePoint 2010 flow
When this is released I plan to implement the feature to allow external email confirmation as a part of a larger approval process flow that will eventually be worked into a power app. Look forward to seeing this status change.
We do plan on adding support for guest users to submit approvals.