As a result of this vocal backlash on social media, Google actually reversed their decision to completely deprecate the Structured Data Testing Tool (SDTT). Instead, they announced in December 2020 that the tool was to be 'refocused' and migrated to a new domain, by April 2021.
This new tool is known as the Schema Markup Validator, and lives on a schema.org subdomain. It became publicly accessible to beta users in May 2021, and officially 'launched' as a SDTT replacement in August 2021, when it came out of beta. At this point Google redirected the old SDTT page to the their new 'test your structured data' page, which is the one that offers the choice between the Rich Results Test and the Schema Markup Validator (image at top of this post).
The Schema Markup Validator itself has a very similar UI to the old SDTT, and works in basically the same way, enter a URL or code snippet. Run the test, and then the results show the extracted HTML on the left, with the validated schema entities on the right.
This tool will check all structured data on the page, but will not validate against Google's guidelines. This means that if you care about Rich Results, you will also need to check using another tool that does validate for Rich Results. To ease the frustration of using multiple tools, we built a handy Schema Markup Validator Bookmarklet, which is worth adding to your browser if you do a lot of structured data validation.