While I do appreciate the addition of the new red-flags tag as it provides me a convenient list of questions to vote to close as opinion-based, I think it's probably in the best interest of the site to not have this as a tag.
Do we need this tag, and what do we believe it is for?
No, red-flags is not a valid or helpful tag and should not be used.
It's a meta tag, per https://stackoverflow.blog/2010/08/07/the-death-of-meta-tags/. It is entirely unsatisfactory as a single tag on a question. It's like having a tag of [problem] or [things-I-dont-like]. It may as well be on almost every single question on the site describing something someone finds negative about a company, employee, candidate, HR department, etc. What does it mean to be an expert in the [red-flags] tag? Nothing. Why would you search on it (other than to find things to fight about, or find opinion-based questions to close?) No reason. It covers bullying bosses and slow recruiters and employees with strange habits - it's a valueless meta tag. Also its meaning is not clear, which I'll cover next.
Its definition is not uniform across its users. What does it mean to say something's a "red flag?" Something that's negative at all? Something that's so negative you shouldn't work there/employ that person/etc? Rather than encouraging people to use their words as to what their actual concern is, it encourages people to use a poorly defined/scoped term of "red flag" and move on as if they'd actually communicated.
And this is closely related to the third and fatal problem - using the term "red flag" is a very accurate marker for opinion based questions on the site. "What are issues to look out for in a candidate that spent 15 years in their previous job" is a good question. "Is it a red flag that a candidate spent 15 years in a previous job" is a crap question. It's looking for a yes or no answer, and that yes or no answer is a purely individual opinion/value judgement. People may add useful information to their answer, but through no benefit of the question - or of the [red-flags] tag associated with it.
I propose @Lillienthal stop adding this tag to questions and burninate it instead.
While I don't think this will always translate as meaning the underlying question will be opinion-based I do think it's a bad tag.
Primarily because it's a horribly over-used, too-broad buzzphrase - people throw it around as though it actually means something but can never explain just what it means or if they can it never seems to agree with what other definitions people have given. Which, effectively makes it mean nothing and making it into a tag does nothing to resolve that issue, and just makes it worse.
