Thank you both!
Wow, @nathank with your advice, I think I have finally found our welcome post, which has been both edited from the original and deleted!
And @NateDhaliwal your explanation made me realise that our site may be different from others in that our home page displays our categories for browsing, not the Latest topic list with the pins on the top. That makes me understand why that topic really would be clearly visible with the same kind of home page as this meta site has (like in the screenshot below) but would require a user on our site to go somewhere (Latest topics) to see it.
That is super helful and gives us enough info to think about how we want to work with this welcome topic option. 