With people shifting their layouts over to Flexbox, relying on "box-sizing: border-box", "display: table-xxx" and friends, older browsers will mess up the layout beyond belief. IIRC some older browsers even failed to apply an entire CSS file if they couldn't parse a single rule - good luck with ::before, ::after, ::nth-child and friends.
CSS, unfortunately, isn't really backwards compatible unless you invest a LOT of effort into testing and development.
Good riddance. Those older browsers are also going to be full of unpatched security holes, and if websites don't work people will eventually quit using them.
Many recently updated websites of unicorns/startups look very bad or often completely unreadable on older browsers e.g. IE 9. Can they live without enterprise sales?
CSS, unfortunately, isn't really backwards compatible unless you invest a LOT of effort into testing and development.