This is certainly done with COVID-19 in mind. Is there any source that shows how effective HEPA filters are for filtering SARS-CoV-2, given that its average size is just 130nm, which is twenty times smaller than PM2.5.
0.3 micron was chosen because as noted by another commenter below, high efficiency filters typically filter better at smaller sizes than this.
A more stringent certification is EN1822, which first tests which particle size penetrates the filter the most, and then rates the efficiency based on that particle size.
With respect to viruses, a minimum of EN1822 H14 is required for use in European biological safety cabinets. An H14 filter is 99.995% efficient as it's most penetrating particle size (MPPS).
That said, it's important to realize the difference between using a filter to pass air into/out of a clean environment (e.g positive/negative pressure clean room) versus just using an air purifier in a room. In the latter case, the machine is only going to filter the air going through it, and will take a significant amount of time to turn over the air in the room, during which time it's just blowing things around.