It seems that privately created or published homebrew is shared in a campaign with content sharing enabled, unless it's a monster. Backgrounds, feats, spells and magical items automatically get shared, but monsters do not. I'm guessing that races and sub-classes will automatically be shared like most other things but figured I'd ask anyway, while I was asking if monsters will ever also be shared?

I don't know if it's odd or not, but I share monsters with my players all the time. Mostly to get them to run them in combat as allies, like with Felgolos or the like.

Currently the sharing logic is tied to things that can be applied to a character. Eventually, that will include monsters when either wildshapes/companions make it into the sheet, or the campaign management tools start getting built out. Whichever comes first.

I hope that eventuality also comes with a toggle option. :p

I find myself keeping items and spells in notes rather than make them actual homebrew until the party actually acquires them, to avoid potential spoilers.