Workspace owners and admins can manage all the channels of their workspace, excluding the personal ones (a.k.a. "My private channel".)
As a workspace admin or owner, you can head to your workspace settings and select "Channels" on the left-hand side. From there, you'll see a list of existing channels, including the ones you have not been invited to.
At the top of this section and in the security settings, you can manage who can create new channels in the workspace:
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From the channels list you can delete channels, or reassign ownership to yourself. This last option can be helpful if a channel admin leaves the company.
The workspace admin can then (temporarily) take over the user management of this channel and make sure it's shared with the right people within the organization again.
To do so, click the three-dot menu on the right, and choose "Open settings." Then, you can manage the channel from its Share modal that will open automatically.
The Last activity sorting option at the top can help you with the maintenance of your account. For channels that have not been active in the last year, it can make sense to delete or archive them.