We can use Heroku as a forever-free sandbox solution for testing or hosting micro service. Adding a credit card to have 1000 free computing hours.
Heroku will make a service down if no received request come. We can use a cronjob-like service to check service health and keep it live !!! ;)
Cronjob check health SASS : pingdom, statuscake, port-monitor, uptimerobot
Btw, I don't recommend you keep service live but no use, it makes Heroku infrastructure heavy and THAT'S NOT FAIR for them !