Comment 3 for bug 1829588

Its worth mentioning that software-properties-gtk has a "Developer Options" tab that allows you to enabled and disabled -proposed (provided you aren't using a local mirror of the archive) and that is the preferred method to enable -proposed. Which wiki page were you looking at? It probably needs updating.

Additionally I briefly looked at the code for add-apt-repository and it seems to have support for enabling / disabling components (e.g. multiverse) but not pockets (e.g. -proposed).