Does it makes sense to have private channels in an open source community? If so, which channels should be private and why?

Usually when the people in the channel are invited basis. E.g. Pinot-committers or pinot-pmc. There are also certain channels for developing features or integrations, the purpose is to not make a lot of friction.

What type of friction would be made if those private discussions were in a public channel (#C013WKLT5T7)?

Most concern is for confidential stuffs and that’s rare. I would say majorly such discussion should happen in github issues or the channel.