Please Please Please add it!
This would provide a way to create branding without customizing SharePoint masterpages
Thank you for your input around this. We have analyzed this request internally and due the direction where we are heading with modern experiences, we won’t be exposing this API in the CSOM side. There will be enhanced options to control branding for the modern team sites. With classic team sites, you should be able to achieve typical scenarios with UserCustomActions or alternatively with custom master pages. We do acknowledge that the API would be useful, but it could also open up additional challenges for the future.