Since you mentioned that this channel is the default channel that was created when you set up Mattermost for the first time:
This is auto generate channel when first setup
I would think that this channel behaves like the default Town Square channel:
which does not have the leave channel option since it is meant to be the single channel where every user in the Mattermost team can collaborate together.
If you are looking having the feature to leave the channel, we do not have any specific feature available to do so. This topic has been brought up to the team here: