Modern HTTP Protocols Health Check failing
Platform Notice: Server and Data Center Only - This article only applies to Atlassian products on the server and data center platforms.
When running the health checks from General Configuration → Troubleshooting and Support Tools, the Modern HTTP Protocols check will fail unexpectedly.
Atlassian Troubleshooting and Support Tools (ATST) ver 1.30.1
CONFSERVER-58307 is the product suggestion to implement HTTP2 in Confluence, the suggestion notates a number of concerns with implementing HTTP2.
There are a lot of infrastructure concerns that have to be addressed to establish the end to end HTTP2 connection; even connections made directly to the application, bypassing any proxies or load balancers, may still not make HTTP2 connection natively without adjustment to the Tomcat web server and SSL.
In this case the failing health check was a false flag caused by a bug in version 1.31.10 of the ATST addon. A similar environment running the same version of Confluence was not having any health checks fail, it was discovered the affected instance's ATST addon was out of date.
Upgrade the Atlassian Troubleshooting and Support Tools addon to latest. The Universal Plugin Manager should suggest the latest version for you.