Bear in mind, the new web+app properties are in beta, a few people of note in the Analytics and Tag Manager community have made a point of stating this type of property is nowhere yet near close for using in a production environment.
Custom dimensions and metrics do not appear to be available. From the reading I have done so far, I'm guessing they may get replaced with Custom Parameter Reporting
You can also identify up to 100 custom event parameters per project
(50 numeric and 50 textual) to include in reporting by registering
those parameters with their corresponding events. Once you register
your custom parameters, Google Analytics for Firebase displays a
corresponding data card in each related event-detail report.
I haven't looked into this much in relation to setting it up. However GA does provide space in it's new reports for this type of property in relation to custom parameter reporting. eta.. cant find the documentation i was looking for however i think it is 20 text and 20 numeric for GA standard