Our adult content expert is currently on vacation, so it might take a little while before you get a proper response. But, just do you know, we can only help you if you truly believe it has been incorrectly marked as an Adult Movie as per our guidelines. Otherwise, this is something Letterboxd has to fix on their end.

lineker set it that way as adult and so I think it's correct

The problem with creating more exceptions for porn is that eventually we would have most classical porn films available for all users to see on TMDb and that is not a good solution. I still think the easiest solution would be for Letterboxd to create an alternative system for amateur content, classic porn and some video entries that would make those titles available on LB without users having to break our rules here.

But why would it be an issue to change the adult setting? Well, recently a third party developer had their apps removed from an App store because of one title where a user forgot to set the entry to adult:true and where graphic images were added. One title. Imagine what would happen if there were hundreds (I know the user only proposed a handful of titles, but why stop there?). In a way, the adult:true setting is also protection for third party sites and developers. So, any decision of this magnitude would have to be made by Travis and I doubt this is something he wants.

Finally, I just wanted to make sure that you all know that this policy is not in any way a case against the artistic qualities of classic porn. It's just a setting to differentiate porn from regular movies. I still think the best way to resolve this is what LB actually proposed in that 2016 help topic. But I can also understand the problems of making sure to not import hardcore images into LB. It seems some users in that topic want to import any hardcore title with a current physical release, which is a lot more than just selecting a handful of important titles to import.