Maintenance Mode in SharePoint Online and Modern pages

Turns out, if you append ?maintenancemode=true of your SharePoint page you will see a special maintenance view:

On your page, you will see a summary of data from the Manifest and Data tabs. The summary information includes:

Alias	The name of the web part
Id	The unique ID of the web part
Instance Id	The ID of a specific instance of a web part (that is, if you have two more of the same web parts on a page, they will each have the same web part ID, but a different instance ID.
IsInternal	Indicates whether the web part was made by Microsoft or a third party. If True, it is made by Microsoft. If False, it is made by a third party.
Version	The version number of the web part.
Environment	Environment: Indicates the SharePoint environment in use. 0 = Test environment 1 = Local Workbench 2 = SharePoint 3 = Classic SharePoint
UserAgent	A string that contains information about the device and software in use (such as browser type and version).

Microsoft article "Open and use the web part maintenance page"