“the MIT License also permits reuse within proprietary software, provided that all copies of the software or its substantial portions include a copy of the terms of the MIT License and also a copyright notice” from Wiki - https://en.m.wikipedia.org/wiki/MIT_License
So maybe illegal if the license isn’t in the repo?
It's legally and ethically fine as long as the copyright notice remains, right? I'd simply make a PR comment noting that the copyright notice should be included. They might not know it's needed, and either way lets them save their face. No need for drama unless they make it.
Probably fine - even if it was in breach of some os license - how would they (the original author of the code) know and lift a burden of proof in a civil court?
Besides that it is good practice if you copy something from somewhere to put that fact in a comment or commit message.
Makes it easier to maintain.
I'd just flag that you may want to confirm license requirements in the PR review. It doesn't seem like a rare oversight to make and code reuse is so common in general.
I remember reviewing someone's PR and realizing I recognized some of the code from a previous company (that was open source, but there was no attribution). I just pointed it out in my review and we decided to remove that code, no big deal.