Hi Mark,
Atlas’ Full Text Search feature (beta) builds on Apache Lucene which requires provisioning additional processes that run within an Atlas cluster. Originally this feature was only available on M30 or higher dedicated clusters to ensure adequate performance.
Full Text Search is now available on shared clusters (M0/M2/M5) because the search infrastructure can also be shared for these deployments.
The M10 and M20 dedicated clusters have lower available resources, so there are some performance concerns that we are addresseing before making Full Text Search available for those tiers. There is currently no public date set, but when available this will definitely be widely announced.
You could also raise this as a feature suggestion on the MongoDB Feedback Site so that others can upvote and watch the updates.
Regards,
Stennie