Currently the PowerApps Custom Form in SharePoint will only work if the modern experience is enabled. If the user clicks the "Return to classic SharePoint" in the browser then the out of the box SharePoint list form is used instead of the PowerApps Custom Form. If a PowerApps Custom Form is being used in a list then the Classic toggle link shouldn't prevent the custom form from being displayed.
It is now possible to play a PowerApps Custom Form from the classic experience (if the form has been customized prior to the user switching to classic).
Adding @emmac who will be working to improve these experiences. She and I were able to run a form from classic which had previously been customized by a maker using the modern experience to confirm this change.
Thank you for your patience as we continue to evolve this experience,