I think if you can get it for a good price, it would be worthwhile. It's only a year older than the XR (despite many on this thread acting as though you're thinking of going for a 6 Plus), but will obviously be two years older than the XR's replacement that will probably be due out next month. Still I've run into instances where some decisions on the non-notchy iPhones feels out of place compared with the iPads and notchy phones, so the XR may provide a better experience if you care about that sort of stuff (the battery charging indicator is different for no reason).
If anything, try to wait until after the keynote and prices on the 8 Plus and XR will drop, or you can get the XR's replacement.
Regarding support, the 8 and 8 Plus should have quite a few years left, as the 6 will be dropped in the fall.