Hi @Michal ,
Great to know that and thanks for sharing the update here.
By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.". and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
[Ready only template in SharePoint and Desktop app]
Issue Symptom:
There is a template power point file in a SharePoint library. It has read-only settings. However, if opened in a desktop app it can be overwritten. It does not behave like a template. How this issue can be resolved?
Current status:
"It has been resolved via checking out a read-only file on SharePoint."
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community members to see the useful information when reading this thread.
Thanks for your understanding!