Docs says nothing yet for RHEL 9.x - https://www.dynatrace.com/support/help/technology-support#linux
But I have installed and worked just fine, at least for the technologies I tested (nginx, httpd).
I'd be happy to answer. We tested OneAgent on RHEL 9 right away after its GA. Dynatrace OneAgent support for RHEL 9 will be announced with version 1.245. It will be released by the end of June 2022.
I also have the pleasure to share outcomes of the Dynatrace and Red Hat partnership program that resulted in the official certification of Dynatrace OneAgent on RHEL 9.