I held off adding this for the longest time because I thought it would be difficult, and also part of me felt that buying custom domains is purely a vanity move (and a common addiction for makers, me included). My theory is a custom domain won't help you to validate or invalidate an idea, so it's not necessary.
Nevertheless I had a fair few requests for custom domain support so I decided to add it but make it a paid feature. Right now anything that can make the paid plan more attractive without degrading the core free service is a plus.
It took me a few days to get the process smooth but I managed to ship it within a week and I'm super proud!
To announce the update I made a 2 minute video of me creating a landing page and publishing it to a custom domain, all within 20 minutes.