I am skeptical of an idea…

I am skeptical of an idea that users should post url handlers, but otherwise yes, having a 30k list with the pubkeys of apps I use for kind D which goes to d tag seems like a good idea to efficiently query 'which apps people I follow use to handle this kind'. And url handlers should go to the apps' profile so that only app could change them, otherwise malicious apps could insert random urls to my app list for other apps.