It's not going to be significantly unsafe. It'll shed more microplastics and is more likely to grow mold due to the surface texture, but if you're inhaling through the whistle you're using it very wrong.

That said, an injection molded whistle can be bought by the thousand for a few cents each.

Polylactic Acid (aka PLA) is a protein found in milk and corn. PLA filament is made from fermented corn starch.

If you would rather invest in a quality whistle for the one that goes in your own mouth, I recommend an ACME Thunderer.

As used by gym teachers and drum majors, (not Wile E. Coyote) they have a satisfying roll of authority.

The biggest concern with these come from the fact that it's hard to decontaminate 3d printed stuff due to the texture, as far as I can tell? PLA, and PETG are food safe, but 3d printing does leave a texture that's hard to fully clean, which can cause problems over time.

For microplastics: You're getting a lot from a lot of sources, and as with anything it's an "Is the risk worth the benefit" situation