The /page/mobile-sections/{title} REST API endpoint is deprecated (as part of the mobile content service that is deprecated).
However, inside ContentTranslation extension codebase, this endpoint is used to fetch the section titles of the target page, when a section translation is published. Thus, following the deprecation, a different way of fetching the target section titles has to be found and used to replace this usage.
Additionally, a patch pending for review, also uses an endpoint from the same service in a similar way, and should also be fixed.