In the past I believe it wasn’t available because of needing to declare the nationality of the investor purchasing the shares, post here mentions it - International Consolidated Airlines Group - IAG - #2 by gt94sss2
There are a few shares like that for national security reasons, but that was fixed and IAG was available for a short time.
But it then got removed a couple years ago - International Consolidated Airlines Group - IAG - #8
Apparently something to do with the Spanish tax authorities and how investors here would have to pay tax, not entirely clear, but doesn’t seem like it’ll come back anytime soon nor would Spanish stocks be listed here on the same basis (not that you’d want to buy them, one of worst performing markets in Europe)