I think this had less to do with Let's Encrypt, and more to do with the fact that StartCom's root certificates were distrusted by nearly all major browser vendo

I think this had less to do with Let's Encrypt, and more to do with the fact that StartCom's root certificates were distrusted by nearly all major browser vendors some time ago due to [their parent company's violations of the Baseline Requirements](https://wiki.mozilla.org/CA:WoSign_Issues https://wiki.mozilla.org/CA:WoSign_Issues).