I create websites for the healthcare industry. I know exactly what I'm saying. There are some clients that I still need to write IE7 fallbacks for - and a single client that I still write IE6 fallbacks for. Lucky for me our contract has dropped IE7-9 support by default unless it is specifically requested by the client.

Apologies, but offhand I can only think of flexbox issues (specifically related to wrap/min-width, which has been fixed in version 11) and with interactive form validation (which was only just fixed in version 10 or 10.1). I run into various issues when testing on iPhones and the largest section of edits caught in any internal QC are always iPhone/Safari related. With relative certainty nowadays - if it works in Firefox, it works in Chrome and Edge. But then I always need to write some workaround or another for Safari.

The biggest issue is that even when these bugs are fixed - I still need to support them for an extended period of time because for whatever godforsaken reason, Safari updates are tied together with operating system updates. Want the newest Safari? Buy a new iPhone. It's the Windows XP/IE8 problem. Where IE8 had to be supported only because many users were still using XP and couldn't upgrade to IE9 even if they wanted to.