I'm wondering when that was ever true? Until the advent of the iphone, from my recollection phone devices didn't allow you to install whatever you wanted. In fact they were even more restrictive for developers than they are now
Tons of counterexamples: Windows Mobile, PalmOS, Symbian, even dumbphone operating systems like S30 allowed J2EE apps. Of these, the strictest control was exercised in the J2EE ecosystem, which could be (and often was) configured such that only programs signed by the OEM could run. But that was an extreme case, because all smartphones operated with what kids these days called "sideloading." "Sideloading" used to be called "installing" and you didn't need to do it with a specific piece of software run by the company who issued your operating system.