[roskelld]

I don't know if Discord fixed it as I haven't checked in a few years, but I tinkered with scraping some public Discords and I found that I could see hidden channels, not the data, but the channel names, which could do things like reveal to me if the same Discord was used for in-house development if it was a product Discord. Not great.

[0xC0ncord]

This is still the case. There are even some client mods that let you view hidden channel names and know what roles/permissions are required to participate in them.

[tuetuopay]

You can still see them. Using alternate clients you will see them, and bots also see them.

[judge2020]

This is technically the case - I believe the existence of private channels is still sent to the client (eg. their snowflake IDs, which also reveal creation date) but the channel names are no longer sent as well.