I'd like to hide the '+ Add Column' button:
On Sharepoint 2013 On Premises, I would have just used some CSS in a script editor webpart to hide the unwanted fields / page features. There's no option to add this in Sharepoint Online.
Kind of related questions, but not identical:
How to hide "Share", "Copy Link", "Copy to" buttons in my site?
Unfortunately, + Add Column button in SharePoint online modern experience list view cannot be hidden using SharePoint out of the box settings.
Even it cannot be hidden using SharePoint JSON formatting.
If users are having Edit or higher permissions on the SharePoint list, they will see this option.
You can grant Contribute access to users so that they can add/edit/delete list items.
However, they will still see the + Add Column button in the list view. But, they will not be able to create new columns in the list. They can only see these options currently:
If you want Microsoft to provide functionality/settings to hide this button in the list view, you can add a feedback/idea on SharePoint feedback portal: SharePoint feedback portal
