The OptaPlanner REST API documentation disappeared as of 7.15.0.Final.
This section is what's missing, 7.14.0.Final: https://docs.optaplanner.org/7.14.0.Final/optaplanner-wb-es-docs/html_single/#_optaplanner_rest_api

The required docs on github: https://github.com/kiegroup/kie-docs/blob/master/doc-content/shared-kie-docs/src/main/asciidoc/KieServer/kie-server-rest-api-solvers-ref.adoc