Thank you very much for this complete information. I often use one of the STM32duino.com targets.
And I wait for a major update of this package, this would be very welcome. I already posted about my confusion with STM32 targets, because of the 2 different packages.
At a quick look I didn't see the FreeRTOS in the STM32duino package so I believe it's caused by https://github.com/rogerclarkmelbourne/Arduino_STM32 but I could be wrong. This only happens when you have the board of a hardware package with a problematic library selected in the Tools > Board menu. The updatable notification also comes after some delay, so it can be confusing to determine why it happens sometimes and not other times.
I should have had the idea to add a properties file to the RTOS library myself; this hint is highly welcome.
I'm glad if it is helpful. Let me know if you have any questions about how to do that. The exact value of most of the properties in the file are not important so you can just copy one from another library and change a couple lines.
I didn't know the accurate information that should be filled in for that library in the public distribution so that's why I opened that issue report about it rather than submitting a pull request, I figured the repository maintainer would know and thus was better qualified for the task.