I think it's highly dependent on which airport you're at and the current mood of the people working there.

Last year I was flying home, and there was a sign in the security line saying they we don't need to remove our shoes or belt, I cannot remember which. When I got up to the front of line, I was told, yes, I need to remove both.

For CLEAR, PreCheck and airline-specific special class passengers you can skip removing shoes/belt/jacket, depending on the policy, when traveling through US airports. Non-US airports, and certain international destinations, don't require them removed. And sometimes they just don't give a shit and will let you through regardless.

I think it depends on which TSA agents you get, and what kind of mood they are in

This. I've been instructed to take my belt off in the dozen or so domestic flights I've taken the last ~2 years. Before that it was hit or miss.

I remember this because I'm thin and a belt keeps my pants from falling to my knees while I'm holding my boarding pass, ID and raising my arms above my head so my body can be scanned with questionable technology.

The scanner will flag it up. I left headphones in my pocket; they were flagged. I got rained on before entering the airport; my damp clothes were flagged. This will make you subject to a mandatory groping at the very least.

Yeah, the scanners are very sensitive and will highlight any abnormality. When I flew earlier this month, the TSA agents were walking up and down the line announcing "Empty your pockets completely, including all tissues, papers, and lint; they will show up on the scan". This is true and has happened to me multiple times.

I was waiting outside the bathroom while some TSA agents were chatting. They were talking about how very skinny people with protruding bones set off the body scanner.

I switched to a totally nonmetallic belt and they always have me take it off.

I also always get a patdown and hands run on the inside of my pants waistline. I think this is because I have long hair and the scanner operator punches the wrong button for sex.