According to the US Census website you are required “by law” to fill out the long form if you are selected.
That'll be the day. "Gee, didn't get it." "Uh, gee, we're really having delivery problems with our USPS lately. Didn't get the resent form, either." "Sure, leave it here and I'll send it in when I get around to it". "Oops, dang dog, why'd you rip that paper up? Now I can't send it in".
But that’s not the long form - there isn’t one this year. That’s the ‘American Community Survey’ which is sent out to random households in non-census years.
I never heard back from the folk at the census department after I threw mine away.