The latest version is 10.41.06 and you can upgrade directly to it from 9.21.19 if you want to but would have to request a Wireless Upgrade License Key because you'd be upgrading between two major release versions (v9 to v10) (Links Below):
https://gtacknowledge.extremenetworks.com/articles/Solution/How-to-obtain-wireless-license-upgrade-k...
https://gtacknowledge.extremenetworks.com/articles/How_To/How-To-Redeem-IdentiFi-Wireless-Controller...
The Release Notes for 10.41.06 are available at our website for a full list of new features and fixes ... but in brief some of the highlights are:
Introduces AP3917i/AP3917e/AP3917k (2x2 11ac wave 2) Access Point
Introduces AP3912/AP3915/AP3916/AP3917Thread™ Gateway for IOT
AP3900 Series Integration with AirDefense 9.5
AP3900 Series Integration with ExtremeLocation 1.1
Introduces AP3915i/AP3915e (2x2 11ac wave 2) Access Point
Introduces C5215 Wireless Controller Platform
Fabric Attach: Edge Automation
AP Upgrade with Minimized Service Impact
ExtremeAnalytics™ Signature Synchronization
AP Diagnostics
Auto Channel Width Selection (ACWS)
DFS – Return to Original Channel
Airtime Fairness Update
IOT: iBeacon RTLS (listening) Reporting
AP web server configured with commercial certificates to avoid captive portal browser HTTPS warning
Layer 7 Application signature updates for V10.41
