[mo] Contractor From 10+ Years Ago Threatening To Send Letters To My Linkedin Contacts Unless I Pay A Disputed Bill Within 24 Hours

Location: Missouri

Over a decade ago I was building a custom house in Missouri. I hired a sprinkler company to install the irrigation system, which I needed finished before the city would issue the occupancy permit.

The owner delayed so long that the yard developed ruts from weather, and I ended up paying him extra to regrade the lot before he could start. When he finally began the install, he said it would take one day. He didn't finish that day, said he couldn't come back the next day because he was leaving on vacation, and walked off the job even though we told him we needed it done for the inspection. We'd already given notice at our apartment, so this put our housing at risk. The city gave us a delay and let us move in. He never came back after his vacation. I had to hire a second company that started from scratch.

I'd paid him several installments but not the full contract. He signed a lien waiver when he cashed the first installment. He claims I still owe him for the partial work. I dispute that and told him so years ago. He's had collection agencies call over the years; I told them to contact my lawyer and never heard back.

This week he texted me out of the blue asking for about $X, then offering to settle for roughly 60% of that with a payment link. A few hours later, with no reply from me, he sent this (names and amounts redacted, otherwise verbatim):

"Well I don't want to sound threatening or anything but just FYI number one we're owed the money number two you're the only customer in 25 years that owes me money so on that note since I haven't heard from you and I haven't seen the money I'm going to give you 24 hours to get the payment made if I haven't received it I will have to draw up a letter which remember I do not want to do you're forcing my hand on this but I will send that letter out to several people like [name] [name] [name] [name] etc. and I know that it wouldn't be worth [~60% figure] to me to go through that so hopefully it's not worth it to you"

The four names appear to be my LinkedIn connections.

Questions:

1. Is a claim this old (10+ years, Missouri) still enforceable?
2. Is threatening to contact my professional contacts to pressure payment legal? Does it change anything that he's the original creditor and not a third-party collector?
3. Should I respond at all, or just stay silent? I don't want to say anything that revives the debt.
4. If he does send the letters, what are my options?