Now they do. A Chromebook of the 2015 vintage, to compare apples to apples with the 2015 iMac OP is posting about, stopped receiving automatic updates in 2021, according to Google:Google has 10 year updates on their Chromebooks.
But Google says older machines can receive "extended updates," to bring it to 10 years of support, if the machine's administrator opts in to that program. I don't know why they'd do that, if they're going to provide 10 years of updates anyway, unless it was to cut down on the number of machines getting support. Makes no sense to me.