I know we would love to have this. We work on an older game and we use the CDN to deliver content patches and install images of the game. However, we would like to be able to lock these down to authenticated users. 

Currently this causes us to have to split our spaces up into a non-CDN space for the internal stuff that requires a signed URL, and a CDN space for the public stuff where having one would be convenient but not possible currently. It would be nice to be able to combine them into one space, and have our internal stuff on the CDN as well. Though even if we don't combine them, for separation reasons, it'd still be nice to be able to put them on the CDN with signed URLs.

This is a dealbreaker for us!

I would like to see this feature as well. The docs say "use a CDN" if you run into the rate limit (200 requests per second). But signed URLs are not compatible with the DO CDN. So basically that does mean you are unable to scale private URLs on DO spaces.

Due to the rate limits imposed by DO Spaces, I opted for the CDN, with a custom subdomain (loving this feature btw).

However, I use presigned urls for my private bucket, thus rendering the CDN useless...

Yes this would be very useful. I currently am not at the scale where this is a dealbreaker, but it is a theoretical limitation I'll face if I'm lucky enough to get a large enough user base.