you can use kirby with heroku but you need to load all pages from a db connection as virtual pages. also things like cache and users will still be not persistent.
all uploads must go into that db or things like s3. kirby does not have a file abstration layer built in so you can (an probably never in k3) use stuff like s3 for content easily. there was some discussion on discord about a fork that might add file layer abstraction s3 support.