Back in September, Google added support for uploading image watermarks in Docs and is now making it easy to create your own background text identifiers.
Insert > Watermark on the web will open a side panel that features the new Text tab next to Image. You just Insert text with the ability to customize the font, size (auto available), bold/italics/underline, transparency, and diagonal or horizontal positioning.
Text watermarks will repeat on every page on your document, making it useful for indicating file status, such as “Confidential” or “Draft” before sharing more broadly, no matter the application you use.
You can still “insert an image watermark or images above or behind text.” Meanwhile, Google Docs will preserve text watermarks when importing or exporting Microsoft Word files.
This feature is still rolling out and will be fully available in the coming weeks for “all Google Workspace customers, as well as G Suite Basic and Business customers.” It’s not yet live on accounts we checked today.