Its probably been asked before but can I use a 10 speed rear derailleur with 11 speed shifters and 11 speed cassette on the road bike??

I want to use a 6700 Ultegra mech with 5800 11 speed shifters and 11 speed cassette.

no

No

Interesting, how have they changed the mech?

I knew there’s variable spacing between cogs but not that the cable pull was changed.

I think the pull ratio is different between 10 & 11 speed. The amount of cable moved by the shifter may well be the same, but the amount of lateral (sideways) movement of the rear mech is different.

It looks well different on the front TBF.

I’ve actually tried an 11sp mech on a 10sp system and no, it doesn’t work. Customer brought a bike in with the mech and I didn’t spot it until I’d cabled it up.
Pull ratios are different.
Also, if you’re running 10sp Di2, be afraid, very afraid, because it’s very quickly become obsolete and parts are no longer available new…. So if you smash a mech you could be in trouble….

But with Di2, 10 can be programmed to run 11 & XTR Di2 will run off a road shifter, so I don’t see why an 11 speed mech can’t run 10. Although I haven’t tried that yet as haven’t had chance.