I'm fairly sure those bins have been there since the start of S3.
If you look at
this article of RPS about Apex Legends S3 from October 14th, you can clearly see those bins being placed on the loot bins map:
Now, whether or not those bins always delivered at the very least a Body Shield level 3 since the start of S3, or since the last update, I don't know. I'd be tempted to say the former, since the bins themselves have physically always been there, it's just that it took us a month to catch up on it. I immediately had the idea to go back to that spot over and over again and test it as soon as I discovered it as I had a hunch about it, and it kept delivering. I suppose most people never tested it as much as I did, and never came to the same conclusion as I did, that it was in fact always delivering level 3 body shields all the time. That's probably how it remained undercover for so long: people opening those bins probably thought they got lucky while opening them, and seldom had the opportunity to land at that spot again in their following games, thus forgetting about it, or thinking it was just an anomaly while it's a constant.
