Purchased iPhone X several months ago. Turned on my old 6+ to get pics, and a couple days later received email from Apple saying my ID was locked due to sign-on from unknown device and unusual activity. Is this legit?

Will powering on my previous iPhone prompt Apple to lock my ID?

:No, it's a phishing email :

Phishing and similar emails : Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Genuine iTunes emails : Identifying legitimate emails from the iTunes Store - Apple Support