I'm not an expert of this code.
But IMO it'd be better to return all channel ids here, and just skip the defaults.
I.e. to process logic like this:

        ids[i] = (struct channel_id_and_info){
            .id = ref->cid,
            .defaults = (struct _channeldefaults) {
                .unboundop = -1,
                .fallback = -1,
            },
        };
        if (ref->chan != NULL) {
            ids[i].defaults = ref->chan->defaults;
        }

For example, you can see clean_up_channels method at Lib/test/test__interpchannels.py:

def clean_up_channels():
    for cid, _, _ in _channels.list_all():
        try:
            _channels.destroy(cid)
        except _channels.ChannelNotFoundError:
            pass  # already destroyed

This method assumes that ids of closed channels present in _channels.list_all().

list_all

follow the doc

I think list_all return active channels so do not return closed channels is right.

PyDoc_STRVAR(channelsmod_list_all_doc,
"channel_list_all() -> [cid]\n\
\n\
Return the list of all IDs for active channels.");

I'm not sure about it. We need someone else's opinion.

cc @ericsnowcurrently