I have tried to get the app “usageDirect” running on iodéOS 6.0 on FP4. When I do so, I need to give the app access to usage stats (“System-Nutzungsstatistik”), which is understanable, regarding the function of this app. The slider for granting this greyed out (for all apps which may ask for this permission). If I click on it anyway, I get a message indicating that this may be dangerous (OK, noted) and another message that the app may not function if I don’t (understood). There I also find the text “information of how to give the app access anyway” (German: Informationen dazu, wie du den Zugriff erlaubst), which is blue (probably I hyperlink), but which does not invoke any function if I click on it. My web searches on this issue and on permission.PACKAGE_USAGE_STATS have also been without any success.
I installed the app on my XZ2 with 5.9 running, granted the permission, works fine.
SOLVED: Originally, I did not find out how to grant the app access to usage stats. The normal way did not work. Meanwhile, I have detected the (rather hidden) way of how to do that: In Settings > Apps, I had to select the App, click on permissions (Berechtigungen). So far, I had come the first time, but I did not notice the three vertically aligned dots in the upper right hand corner: there, I was given the opportunity to grant the permissions which were otherwise greyed out.