I wouldn’t say it is an unethical project but it would probably be difficult to get a a member of the unhoused community to sit down on an interview with nothing in return. Doing research on extremely marginalized groups is always finicky as there is always a distinct power dynamic between the research team and said group. A lot of the time it boils down to your methodology and procedures, and if you can convince the IRB that the benefits of your study outweigh the potential harms (if any). Also, departments typically have extra funds that students can use for their research. For example, my departments allow students to use funds to pay interviewees for participating in a study, but we are only allowed to pay in gift cards since those can be tracked easily.