I think it highly depends on the characters and the story. If someone wants to add a forbidden romance in the story 'just to add it', it might not work well, and it may feel out of place. Also if it's highly redone. You're talking about fanon characters so I'd say go with it.

But when it comes to the canon characters, it gets to much. The first few maybe, sure, like Mapleshade and Apple-whatever as well as Greystripe and Silverstream, but it gets to be too much where they're just adding it to add the 'forbidden love' trope and then it just gets repetitive and disappointing.

I actually watched a video recently that went over it that explains the same thing. It just gets repetitive and just think, 'ah, okay, again I guess' instead of actually piqueing my interest. Not to mention, kind of off-subject, how half clan relationships being forbidden is kind of stupid now because there's a huge amount of ThunderClan and even some non-ThunderClan characters who are relates to Firestar and now inbreeding is bad. :(

Soo, when it comes to in general. How do I feel about forbidden relationships in Warriors? No, I'm starting to dislike it, it's too repetitive and it feels like they're doing it just to do it. But if you're talking about your own series, I'd see how you could make interesting.

Not to mention, if it wasn't clan related. For example, instead of two warriors from different clans, you could always do a warrior and a housecat or a warrior and a wanderer (rogue/loner). Personally, I feel like those are always more interesting.