There really isn't such a thing, in the way you've used that phrase.

Capturing telemetry on how I use a tool from within that tool is perfectly fine, to me. Collecting telemetry on my search history in the browser by that same tool isn't. THERE ARE NO INTERIM STEPS that makes the second of those ok. There is no slope. If there is, it isn't slippery. There is a series of discreet decisions and at some point (which is different for everyone) a line is crossed. There was no slope or slip that brought you there, only a series of mostly unrelated decisions.

To think that Microsoft's long-term goal is to install a keystroke logger via a multi-decade and multi-phase plan that begins with application usage telemetry in a free developer tool thanks to "a slippery slope" is just simply not realistic.