Heroku was a product for its time. These days, I see most students use replit.com (in India, at least), including as part of course curricula at universities (paid plans). I'd say replit has since replaced heroku as the getting started tool of choice.
As for heroku, there are many NewCloud companies waiting to pounce: fly.io, deno.com, vercel.com, netifly.com, railway.app, workers.dev some of the popular ones here, while there's also resurgence in packaged / DIY PaaS FOSS alternatives like supabase.com, encore.dev, temporal.io et al.