Miter saw slightly hits plastic kerf when cutting at 45°

I did some research and read that it is OK for the blade to touch the kerf because it will cut through it and shave off the plastic to make a slightly wider clearance.

Yes, that's exactly right. The reason this is generally the way to do this is that the max width of different saw blades varies, some have thicker carbide inserts brazed in place than others which necessitates that the blade makes its own clearance when used in the saw for the first time.

BTW even if the backstop was made from an aluminium extrusion it would be OK to do this because the saw would cut it without difficulty.

I just wanted to see if that is true and if I should just cut and let the blade shave off some plastic from the kerf.

Short answer is yes. But you may want to check first to see if you're exactly at 45° before you do this.

As frustrating as this is, the angle settings on mitre saws (even saws more expensive than the one you've bought) can't be assumed to be perfectly accurate. It's considered good practice to check if you get a perfect 45° (or any other angle) before you do any mitre cutting. If it is accurate that's great, but do check periodically down the line that it continues to give the same accuracy as the mechanism can get a little slack with use.

If you don't happen to have a mitre gauge or a reliable combination square a plastic geometry triangle like you used in school can be used to check your angle here. Despite how cheap these are they're nearly always perfectly accurate.

You might also be able to fold a sheet of paper to give a perfect 45, from one corner to the opposite edge like this: