Edit.
When you first get to the parametric search tool and before you mess with anything on the left, change the default to "SHOW ALL", it looks like it always defaults to New and Popular.
Once that change is made, you can select Pins=6 and the AVR and PICs will come up as appropriate. Interestingly, I see you can select pin count=0 and when I did that, 6 SAM chips showed up - BGAs - I guess they don't consider balls are pins?
BUT it looks like if you simply select All 8 bit MCU, it does not look like it will let you make the "Show All Products" and the lowest pin count=8. You can search for the other sub classes, like AVR and the pin count=6 shows up.
IOW: It's finicky