The Vector Markup Language (VML) format is a legacy format for OLE shape rendering that is not included in the strict version of the ISO standard for Open Office XML formats but is in the transitional documents for backwards compatibility reasons. The DrawingML format is a newer and richer format meant to replace any uses of VML.
PowerPoint has already stopped writing out VML fallback representation for cloud files and will stop writing it out for local files from Version 2205 (16.0.15225.10000) onwards. As a result of this change, add-ins will no longer be able to write out VML and third-party applications only using the fallback VML representation will lose art if they are not updated to implement against DrawingML, which is more modern. Add-in providers and users who use VML should switch to using the DrawingML format.
This change only affects the writing of VML fallback representation; PowerPoint will continue to read the VML format.
A regkey, off by default, is available to allow tenants to reverse this change if needed. The regkey is: Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\PowerPoint\Options\LocalSaveWriteVML = 1