This stuff is right there in Visual C++ 2015 Update 2 which is most definitely NOT a preview product.
There are some comments regarding telemetry_main_invoke_trigger and telemetry_main_return_trigger in vcruntime_internal.h in the CRT source code.
These reference a telemetry.cpp and a telemetrydefault.cpp (neither of which are included in the CRT source)
I can confirm however that the notelemetry.obj file (the source code for that IS included in the CRT source) does exactly what it says on the tin and will disable the telemetry.
I have also disassembled telemetry.obj (compiled form of telemetry.cpp) in IDA and it makes calls to GetLastError, GetModuleFileNameW, __vcrt_EventRegister, __vcrt_EventSetInformation, __vcrt_EventUnregister, and __vcrt_EventWriteTransfer. Source code for the __vcrt functions can be found in winapi_downlevel.cpp (they are functions designed to wrap the real windows API calls with those names so workarounds can be provided on operating systems that dont support them)
So whatever this "telemetry" actually does, it uses EventRegister, EventSetInformation, EventUnregister and EventWriteTransfer to do it (part of the "event tracing for windows" APIs)
If Microsoft wants to silence critics on this they should publish telemetry.cpp and let people see for themselves what it really does and why it doesn't do anything untoward.