We don't currently enable customization for this site...but it's something we are evaluating in our backlog. This site serves as the "back bone" of Hub--a place to see all content created across your organization in ArcGIS Online. The contents of this site is everything you've shared--analogous to looking at yourorg.maps.arcgis.com - if you don't want it to be visible you can disable anonymous access for your org but I strongly advise against this.
It's worth noting we don't let google crawl this site--it's simply there as a tool to help you get to content that's already available in your Organization in ArcGIS Online--but in the Hub's User Experience.
