Hello @CindyCFFP,
While this may not answer your questions as to whether Disallow Threaded Replies can be unchecked globally, the following blog post and attached Public Incident Report should explain why this was enabled: Disallow Threaded Replies option in Discussions
Many institutions were negatively impacted by the change to threaded replies and have provided their feedback on how this change was implemented. My institution opened a ticket requesting assistance with disabling the Disallow Threaded Replies in our discussions as we intended for 99% of our discussions to allow users to reply to one another. Canvas Support responded stating that the setting being enabled after the update was intended and they were unable to assist as it aligned with the behavior they described in the blog post above.
My team and I ended up using Canvas Data 2 to look up what Discussions had a discussion_type of 'side_comment' and work with our LMS Admin to use the API to update them to 'threaded'.