Can we use the remote API to select subhaloes based on 'SubhaloFlag'? This quantity can be easily accessed through online JupyterLab. But I do not find a corresponding key via a remote API call.
You're right, that this is missing from the API. The reason is that this field was added in TNG, and is derived after the fact, so it simply didn't exist when the API fields were set up.
Perhaps in the future this can be added, but I can't promise this soon. I would suggest to download the group catalog files (or use the Lab), so that you can make a selection which incorporates SubhaloFlag.