Although legitimate interest does not specifically require consent it DOES require that the subject is informed of the collection of the data.
Also, legitimate interest doesn’t apply to many categories of data otherwise protected by law. e.g. health info
Also, it doesn’t override their other lawful rights, such as the right to be forgotten.
Link? This was not my recollection of it at all.
Would also put Google and other search engines in violation so doesn’t seem right.
https://ico.org.uk/for-organisations/guide-to-data-protectio...
Basically you just need to declare them in you privacy policy and keep a record for compliance.
In terms of LI though it's really complicated and I don't think anyone here on this site is in a position to say for sure if LI applies to what OpenAI is doing. There are arguments from both sides that make sense.