You are correct. Salesforce has changed rules. Guest users can no longer edit or delete records.
I have been able to insert a record but when I try to query that same record I do not get any records.
To recreate go to non signed in SF browser and go to: https://full-dorothy.cs50.force.com/Member
Click on the “Query Temp Person Sign On” button in the center top of screen
I had the user profile wrong for the data objetc.
Yes Bill - this is what the Guest user changes mean. Guest users can create records. But once created ownership transfers to the default internal user, and the Guest User can only Read the resulting record if you have Guest User sharing rules configured. In addition - the guest user will NOT be able to edit any record.
https://docs.skuid.com/latest/v2/en/skuid/deploy/salesforce/guest-user/#why-cant-guest-users-see-my-skuid-page