After the SEO backlash around Google announcing it would deprecate the tool, Google decided to migrate it instead.
Google said “we’ve heard your feedback” and it is “refocusing the Structured Data Testing Tool and migrating it to a new domain serving the schema.org community by April 2021.”
Rich results tool. The rich results tool will be on Google’s own site, designed and maintained with the purpose of showing you Google Search rich result types. Google said “to test your markup for Google Search rich result types, you can continue to use the Rich Results Test.”
Structured data testing tool. However, Google will be migrating the Structured Data testing tool off of Google’s own domain and on to schema.org. Google said “to better support open standards and development experience, we’re refocusing the Structured Data Testing Tool and migrating it to a new domain serving the schema.org community by April 2021.” Google explained that “the main purpose of the tool will be to check syntax and compliance of markup with schema.org standards.” However, going forward the structured data testing tool will no longer check for Google search rich result types.
Why we care. This seems like an okay compromise where Google will keep both tools under different locations for different purposes. The tool will not go away, as Google originally said, but the purpose of the tool will be for checking syntax and compliance of markup with schema.org standards and not related to how Google shows these as rich results in Google Search.