The one that really annoys me is when they do this for airline tickets.

Airlines release tickets for a flight in batches of 10, which is why you never see a "only x seats left" message where x is greater than 10. Once all 10 are sold, the next bucket of 10 are released at a new price depending on the demand of the previous 10. (I'm sure there's more to it than this, been a little while since I worked in the industry but that's the gist of it).

There's already existing regulations around purchasing flights (in the US). For instance on a flight results page after searching, you have to show the full price with fees, you can't just show the ticket price. I feel like it can't be that hard to regulate the "x seats left" messages too.