A message recently went out outlining this, I wanted to make sure you were aware as it is affecting the modern sites.
Starting over the next couple of weeks we will be rolling out to targeted release customers the ability to support custom columns ( i.e custom metadata) on the site pages library in communication sites. It will also work for the modern pages so there will be no need to create a custom content type for this scenario. Note: There is no UX that will be rolling out with this change.
For users that have been granted SharePoint Online page authoring permissions, this feature will enable them to add columns to modern SharePoint Online pages. Additionally, users will be able to add/edit the values of custom metadata columns of individuals pages via pages library. Additional information can be found on support.office.com.