The legacy service semver.io has been retired
Change effective on 04 October 2022
semver.io was an API for determining available versions of Node.js, Yarn and other software, that has been deprecated since 2019.
The only known Heroku consumers of this service are very old Heroku buildpack versions and/or forked versions of those old buildpacks. If issues are encountered during builds relating to semver.io or semver.herokuapp.com you will need to update to a newer buildpack version.
For any non-buildpack use-cases, a list of alternative data sources for Node.js versions can be found here.