If I remember correctly, heroku has a limit of max 10000 custom domains per app. What are you going to do when you reach that limit, out of curiosity? Thanks
To be honest, we decided to punt on that because if we make it to even 5K custom domains we would be in a great place in terms of MRR, that we could invest the time to find a solution then.
Shard by Heroku accounts?
Yes, we plan to shard by app. Since the cost per app is non-zero we've made it a paid feature. Right now it's on 2 apps (1 to support *.threadbase.io, 1 for custom domains), and we won't have to move to 3 for the foreseeable future. I plan to extract this functionality into a gem :)