Earlier this week the Solr back-end that powers the search functionality of the MusicBrainz website and REST API was migrated to a new cluster running version 9.6.1. This brings a much needed increase in stability and performance, and opens up future possibilities for search feature improvements.
A migration path to SolrCloud 9 will be provided for mirrors as soon as possible. In the meantime we will continue to provide pre-built search indexes compatible with Solr 7. Once a migration path is available the indexes will be provided in the new backup format introduced in Solr 8.9.