Yea, I got tricked into doing something like that once. It is infuriating to see a new feature announced in a software product that's identical to an interview 'homework' assignment you had been given a month prior. "Wow, great solution! However, we've decided to go in a different direction..."
Whether they directly used my solution or gave that problem to several interviewees and picked the implementation they liked best I couldn't know. But I certainly felt used. There's a certain popular piece of software I refuse to use to this day because of my experience interviewing with them (fortunately, they have competition now).
Eventually you get to a point where your gut will tell you whether an 'interview project' is a toy problem or something that might actually be useful.
Whether they directly used my solution or gave that problem to several interviewees and picked the implementation they liked best I couldn't know. But I certainly felt used. There's a certain popular piece of software I refuse to use to this day because of my experience interviewing with them (fortunately, they have competition now).
Eventually you get to a point where your gut will tell you whether an 'interview project' is a toy problem or something that might actually be useful.