only for tele lenses on mirrorslappers to get a stabilized viewfinder image. On mirrorfree IBIS works just as well (in principle, not necessarily every real life implementation). In 2018 lens IS is just a duplication of a costly element in each lens, that could be put into 1 camera body and work for all lenses.
There's just so much in that paragraph that is untrue
Take a Sony A7R3 and Sony GMaster 24-105/4, 70-200/2.8 and 100-400 4.5-5.6 all with OSS.
Take a whole bunch of real-life, hand-held photos with the IS switch "On" and "Off", while leaving IBIS on. Now try it with some adapted EF lenses that have In-lens stabilization. You will most certainly notice that IBIS does not work as well as ILIS, even at portraiture focal lengths.
Let me put it another way. If Sony IBIS worked as well as ILIS, Sony would stop putting image stabilization into their non telephoto lenses. Perhaps there is a way to achieve 4 stops of stabilization in-camera, but it doesn't exist today, and not because camera vendors are trying to squeeze us.
The whole telephoto thing... well, anyways, please just try to shoot some stuff handheld with a telephoto lens with and without in-lens IS and with and without IBIS. Since small movements will take the whole subject out of the image, just imagine trying to compensate for shake. IBIS vs. In-lens for telephoto lenses really has nothing to do with whether or not the camera has a mirror; just IBIS by itself really doesn't do a whole lot at telephoto focal lengths.