Regardless of how it is presented, RDP Client 8 only has an imposed minimum version of Android 4.X required. As far as I know, there is technically no limitation on a higher release of Android beyond hardware compatibility (architecture/CPU). Like other Android apps, it simply was not tested on newer releases of Android and Windows beyond its lifetime/scope of development. So there is always a possibility of future incompatibility.
There are versions of RDP client 10/10 Preview that were compiled with a minimum requirement of Android 8. I believe those versions are 10.0.11.1140 through 10.0.12.1148. I would recommend locating and installing the last APK version (10.0.12.1148) from a reliable and trustworthy site for the most up-to-date version that is still compatible with your hardware. Another possibility is the latest/last version of RDP 8.