As part of the general UI improvements to the special:search page, we are focusing on improving search experience and content discoverability specifically for casual readers on Wikipedias.
As such, we do not want to show thumbnails across other projects on the special:page search as this is not in scope of Search Improvements.
Comment: We also have evidence of usability issue of thumbnails on other projects such as Commons and Wikidata. E.g. https://phabricator.wikimedia.org/T320459
- Users see thumbnails only on Wikipedias
- Thumbnails are disabled on other projects for every namespace except File