We have decided to discontinue related channels on the channel page. Related channels weren't frequently used, and we removed them in May 2019 to focus on other features.
You'll still see a setting for 'Channel recommendations' in your advanced channel settings in Creator Studio Classic. As of 16 May 2019, this setting has no impact, as related channels have been discontinued.
You can still add featured channels to your channel page. Learn more about customising your channel page.