The Map Viewer can only be used to delete records if they have geometry. And you can't even do that from the "Data" tab. There is no good way to do delete them as far as I know, which frustrates and confuses our AGOL members - even GIS pros.
Crowdsource Manager is the only decent template for updating records, and we need it to allow deleting records for some of our workflows -- some are "crowdsource" workflows, and others are just needing a reasonably good interface for updating data. It doesn't make sense to require editors to use a combination of 4 interfaces (Map Viewer, WAB, a template, and the REST endpoint), to do simple data management. Even simple workflows performed by novice AGOL users who create embeddable maps need an easy method for updating their data. - and that includes deleting test data -- the first thing users are confronted with when getting started. This situation has caused many who attempted to use AGOL to revert back to other simple web mapping solutions.