Out of curiosity, what’s the upper limit for zoom where image stabilisation will cease to be effective and you absolutely must use a tripod?
I don’t think there is a upper limit, but the real answer is it depends. Shutter speed, if you subject is moving or not, pixel size, even how close a moving subject is to the camera all play a part on if you’ll get blur or not.
image stabilisation (IS) will help no matter what the zoom, with a tripod helping more. After all, if the camera isn’t moving then there is nothing to stabilise. Even pressing the camera shutter whilst on a tripod is enough to cause small movement.
put simply, the better the IS, the better you can get away without using a tripod. Trying to stop the camera from moving in the first place is of course the main aim, be it by leaning against a wall, resting a camera on a solid surface or by using a tripod.