That's a frustrating situation with your Instagram account getting suspended out of the blue. It sounds like a total head-scratcher, especially since you haven't posted in ages. What a goofy glitch! They're asking you to verify your mobile number through a text, but it's understandable that you feel skeptical about complying. It feels like some shady MITM attack, right?
I failed to follow up on this thread, but 3 or 4 days later, they emailed me back essentially saying, "Oops, mistake" and gave me back my account.