Flex was added in 2009, but it had bugs as late as 2017. It is one of the problems with adding features as fast as they are. There is no point in having features if they don't work or are broadly supported

I think it's not the work of updating but trying to remember.

If you are doing consistent frontend you can probably keep it all, but if you only touch it occasionally, it's just too much.

Yes. Many years ago I had the time to devote to keeping up with changes to CSS, JS, and all the other languages and technologies I use, but now I have increased responsibilities at work, a family that needs my time, a home that needs maintenance, etc. I simply do not have a minute to spare with updating. Maybe after I retire.