Handholding any long lens takes as lot of practice, even years. But with enough time in the field one can develop a technique that works. Me, I handhold everything, excepting long exposure fireworks, my tripods could be used as plant stands. I use the OIS in my PL100-400 instead of the IBIS in my E-M1 II so that I get a steady view even before half press for focus......… the reason is that I also use magnification and peaking to zero in on the critters eye and at 400mm plus 5-7X magnification the image in the viewfinder is just to jumpy if not tamed by the OIS. S-AF +MF usually nails it, unless the damn critter moves.I know that IS is a popular thing, and I love it for regular photography, but with long-lens wildlife photgraphy, I usually use a tripod. It is just very difficult to get precise aim when handholding. I mean, for example, it makes a difference if you focus on a bird's eye or its chest. Usually you don't get the bird full enough in the frame to easily pick one or the other. So you need to kind of shift the aim around, firing away meanwhile and hoping one of them nails it. I cannot imagine doing this handheld. It's hard enough with a tripod.
I suppose if you go to Yellowstone and photograph bison, then you can handhold a long lens, becuase they are BIG, but otherwise, I prefer a tripod (even though they are a big PITA).