To put a 9th gen iPad in Diagnostics Mode, hold the power and home button, plug into power, hold for two seconds after the Apple logo appears. Then the iPad will boot into Diagnostics Mode.
The help article for iPad diagnostics mode only pertains to devices without a home button. The 9th gen has one.
[Edited by Moderator]
Presumably the threads to which you refer were relatively old. Old/stagnant threads that have had no replies for some time are automatically closed by the system.
As for your notes concerning diagnostics mode and older devices, in posting here you will have helped others seeking this information - assuming, of course, that they bother to use the search tools available here. More often than not, people post blindly without first searching for information using the available tools.
How to put 9th Gen into Diagnostics Mode.
