TLDR Sputnik 1 / Explorer 1
Rationale: I believe the terms COTS and hi-rel have taken a while to take hold. If we take the assertions of the article linked to by RUNTIQ as correct then the significance is unstated but can be deduced from this sentence (end of 3rd para)
Telstar had become the first “victim” of radiation-induced electronic
failure. Six other satellites were lost within the next seven months,
and the earnest study of radiation effects in electronics became a top
priority for engineers and scientists the world over.
The apparent implication is that the concept of having specially selected parts of space missions, specifically with respect to radiation tolerance, did not exist at the time of the Telstar 1 launch.
So if that is the inference then why choose Telstar 1? Surely all satellites prior to that time used commercially available parts because no other concept existed at that time. You may note I've generalised away from any special modern definition of COTS to just "commercially available". It might be better to say Explorer 1 rather than Sputnik 1 out of recognition that electronic parts in the Soviet Union may not have been considered "commercial" at all.
Obviously the concept of COTS has many dimensions, not limited just to radiation tolerance. However the twenty years prior to the Telstar 1 launch saw a great deal of evolution of transistor types and the idea of mass production hadn't been around for long. That all said, I'd be surprised if there wasn't already in existence a notion of procuring transistors of different quality, e.g. with respect to repeatable parameters and tolerance to high current and temperature.
Another aspect of this, which would help put an answer together, is when parts stress analysis began as a discipline, as this would very much inform the choice of component sources. My suspicion is this also began in the early '60s and I recall once seeing a paper on the topic dating from 1964 or so, though I can't find a link to it any more.