I know that digital sensors have filters that filter out UV and IR light and thus do not require UV filters, but what about film? Are IR or UV filters required when shooting film?
"Required" is probably a bit of an overstatement, but they are certainly more useful for film shooting.
The exact effects that UV have on your film photographs will depend largely on what film you're using, and the conditions under which you shoot. From the Wikipedia article UV filter:
Historically, some photographic films were sensitive to UV light, which caused haziness or fogginess, particularly with a blue hue. However, newer photographic film and digital cameras are highly insensitive to UV wavelengths.
I bought a used Minolta with UV filters on both lenses. One day while shooting, I took it off half way into a roll of 200 speed film. The shots were super sketchy after that, so if you are shooting outside in bright sun with film, I would recommend it. But, that's just what I think — could be different with different film and different cameras.
The light sensitive goodies in photographic film are naturally only sensitive to ultraviolet, violet, and blue light. These blue only sensitive films yield images that sometimes appear strange. Red objects like lipstick lips and rouged cheeks reproduce dark gray, maybe even black. Photo scientist learned to add dye to the film emulsion. These change the color of silver salts forcing them to absorb green and red light. First came orthochromatic (blue & green sensitive) followed by panchromatic (red – green – blue sensitive. Such treated films never lose their sensitivity to the blue regions of the spectrum.
Under ordinary conditions, the fact that films are highly sensitive to UV, are of no consequence. This is because most camera lenses are made of glass. Glass blocks a high percentage of the UV energy. There are situations when a UV filter can be quite helpful. We are talking about the specialty of imaging objects that exhibit fluorescence when illuminated with UV light. Additionally, mounting a UV filter when doing high-altitude aerial photography. This is because, at high- altitude, the air that normally absorbs UV is scant.
In everyday photography, mounting a UV filter works to cut the distant haze present in mountain vistas. Let me add, the UV filter is not effective for distances under several miles.
In recent times the UV filter has been pushed as insurance against accidental scratching and other perils. This concept gives piece of mind however it mainly serves to line the pockets of suppliers of camera paraphernalia. Modern digital cameras are less susceptible to the effects of UV energy. These too use glass lenses plus the protective cover glass that overlays the image sensor blocks UV and IR radiation.
Let me add, good filters must be optically flat. From a technical viewpoint, making an optical flat is quite difficult. In other words, we mount filters only when the benefit outweighs the bad.