Hi @john john ,
In September 2021, Microsoft changed content type publishing working mechanism to improve performance and make newly published and updated content types available to sites faster. This message is just a notice to end users. you can ignore it. To know how the new content type publishing works, please refer to this article: What’s changed in content type publishing.
The steps in your post of assigning a custom content type to a document library are totally correct. Now there is also a new way to add published content types directly to a list or library:
- Select Add column in the library.
- Select Content type in the menu.
- In the drop-down list, all published content types and custom content types available are listed.
- Select the published content type that you want to add. Select Apply to confirm the addition.
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