My tests showed only about a 1-stop improvement between the EF135/2 and RF135/1.8 when hand-held on an R5. This test was just me, of course, and in one shooting situation, but at a wide range of speeds. We only need the size, weight, expense, and complexity of in-lens IS in lenses likely to be used on the very cheapest cameras which would lack IBIS. Here's the tests including the test results presented in hopefully a very easy-to-compare manner to draw your own conclusions on.
Background I want to understand how long I can hand-hold my lenses. Traditionally we used a rule of thumb: the "reciprocal rule," stating that you should use a shutter speed at least as high as one divided by the focal length. For instance, a 90mm lens would call for a 1/90th second exposure...
www.canonrumors.com