[briskman3000]

Took me a minute to figure out how in the hell to get it right, but this seems to work, at least in my testing:

--Hide the Appearance tab function
local function hideapptab(self)
    CollectionsJournalTab5:SetScript("OnEvent", nil)
    CollectionsJournalTab5:Hide()
end
 
function events:ADDON_LOADED(...)
    if IsAddOnLoaded("Blizzard_Collections") then
        hideapptab(self)
    end
end

Not exactly hooking the OnShow, but it accomplishes the same end result from what I can tell.