Am a keen follower of Microsoft's SharePoint Blog and proud to provide this direct from the Microsoft Tech Community:

Administrators may have started to notice our diagnostics starting to render within the M365 Admin Center support portal for certain text queries as early as December 2018. Please refer to our previous diagnostic blog post How to troubleshoot issues in SharePoint Online and OneDrive with diagnostics

 

Start Diagnostics from New Service RequestStart Diagnostics from New Service Request

 

How to ensure I can use the diagnostic?

From the Microsoft 365 Admin portal select Support > New Service Request and type Diag: Classic View to Modern to render the diagnostic. 

 

What does this diagnostic evaluate?

Evaluates that a view can render in Modern mode.

 

What tenant and SharePoint checks does it complete?

For additional details, see SharePoint Online Lists and Libraries Modern View Compatibility check
and Customizing “modern” lists and libraries

 

Validates that page is compatible with Modern mode.

  • Determines if the page is Modern mode compatible.
  • Determines if XslLink customization is present.
  • Determines if page contains a Special View Type.
  • Determines if a Library has a custom new form page.
  • Determines if the page resides outside of a Library.
  • Determines if Modern mode is disabled at the site collection setting level.
  • Determines if form page fields are valid.
  • Determines if List or Library can be found.
  • Determines if the page contains multiple web parts.
  • Determines if Modern mode is disabled due to an unsupported List or Library type.
  • Determines if Publishing fields are present.
  • Determines if a Classic Publishing view was customized.
  • Determines if Modern mode is disabled due to a List or Library setting.
  • Determines if a form page was customized.
  • Determines if a page was customized.
  • Determines if the form page contains supported fields.
  • Determines if JSLink customization is present.
  • Determines if Business Data Connectivity fields are present.
  • Determines if Modern mode is disabled at the site setting level.
  • Determines if form page control mode is valid.
  • Determines if Task Outcome fields are present.
  • Determines if the Page is a List or Library View. 

Validates various page related properties.

  • Determines if the web page exists.

The above is kindly provided by the Microsoft Tech Community!