I have conducted the flour experiment. Water read 1.001. Water plus a lot of flour read 1.012. After waiting 15 minutes, water plus settled flour read 1.004.
So yeah. Suspended solids can change the gravity. Some of the poodles are holding up the hydrometer... for a little while. Once settled out, they don't anymore.
I could repeat the experiment with yeast, but I'm sure I'd get the same results.
What does this matter? Well... if your beer is as murky as a juicy fresh NEIPA, it might matter. Also if you're measuring gravity in early primary fermentation stage, it might not be so accurate. But any other fully fermented beer style...... no impact at all, I don't think. Maybe 0.001. But not enough to matter. My 1.004 flour stuff settled for 15 minutes was still fairly murky, and that was only 0.003 higher than expected (subtracting 1.001 for the reading in plain water). So.