This is something I've always been missing on e621 as well.

Why can't we get a custom feed from favorite artists/directors?

As far as I'm aware, the only way to do this is searching for "~tom_fischbach ~lockworkorange ~ruaidri ~chunie ~zaush .....".
The biggest problem with this is that you can only get so many artists into the search string before it's too long.
So, you'd have to subdivide and manage multiple lists of artists, copy and paste the strings into the search and hardly be able to search for any additional tags inside those "pools".

If I see a "director" has similar taste as I do, I want to see more of their posts.
With all the great functionality of the database system, this seems like a rather basic thing to have.
Please don't tell me there has been a feature for it all along and I just haven't found it...

+1 to this. This doesn't seem like a trivial ask, but it would be a feature I'd use.

+1 here as well, I would love to see this on both sites if that's possible.

By the way, I wouldn't track and display the subscriber count openly or it will just turn into another penis size contest - just for personalized feeds, that's the idea, like private/public "artist sets".