Is International Standard Winding Temperature Trip Necessary?

I think there is a welter of national standards.

CSA and UL rule for houses and factories. Bureau of mines has one for mining.

I expect the VDE or something like it covers the ECM

Each country uses these standards as non tariff barriers, so there will not be a universal one.

http://www.vde.com/vde_en/topportal/standards/

I do not know of a standard for winding temperature trips. There is a wide variation in motor design with respect to the type and grade of the insulation in the stator, for this reason a standard temperature setting is difficult to establish. Best practice is to start motor, allow it run at full load for a few hours or until there is no more rise in stator temperature and then set the trip point at 15 or 20 % higher. take into account worst case air and cooling water temperature.

Some manufacturers will supply exact setpoints for allowable temperatures and vibration and others will gather normal run data and then set the trips based on the normal run values.

It is not un-common to see large motors with only an alarm for stator windings.

It depends on the level of protection you need verus the risk and cost of shutdown due to RTD failure. You can also set it up as a two out of three voting so one sensor failure does not cause a shutdown.

To sum it all up, without a standard you can do whatever you want.