The definition of some plugs is rubbery, a bit like consultant's reports, shop around until you find the one that suits. Wikipedia says that RJ9 and 10 aren't standards, i.e. they're urban myth, don't like that definition, just modify the wiki entry, it's that easy. We've been buying and using 4P4C plugs for 3 decades now, most of our suppliers sell them as RJ11, all of my associates in the telephony industry know them as RJ11. Altronics list them as RJ10 possibly because the Chinese they buy from market them as such. Here's one suppliers definition, it is what most in the industry regard as correct.
https://www.ji.com.au/telecomsplugs/