I completely agree, all of Custom Header should really already be in the core.
It does, howvever, look like @mayker is working on adding some (all?) of the functionality into the core HASS, as he has already for the compact header (thanks!).
At the bottom of the “farewell thread”, he also gives some suggestions for workarounds for some things (I’m still hoping that show/hide entire dashboards on a per-user basis will come to the core, not just show/hide tabs/views).