First, you gotta figure out who is coming for you. Are they reasonable people or cult-minded crazies?
If reasonable, explain. Privately. Like you said, water under the bridge.
If crazies, DO NOT APOLOGIZE. Ever. Do not even acknowledge, especially not in writing. They will screenshot everything. They feed on forcing people to their knees. Do not genuflect or thank them for "educating you". Do not taunt them. Just continue like they said nothing. This will drive them insane (they need to know they are "making a difference") and eventually they will do something so bizarre that nobody will believe them anymore. Play the long game.
What do the crazies know about you? Do they know where you live and work? If they are targeting your employment, and your employer is reasonable, go directly to your employer and explain. They are the people who can decide to fire you. Everyone else can just make noise. So warn them that a crazy train is incoming and ask to meet. In the meeting (do not do this in writing), neutralize the crazy arguments. Give your eight year old Tweet context. Show them how insane the accuser is. Explain that you're willing to make amends/formally apologize/undergo training if anyone in your workplace "feels unsafe". Give your boss some way to "handle" you internally that will placate the crazies, but will still leave you employed. Lawyer up, just in case.
If your employer is unreasonable, lawyer up and start making backup plans for income. You're in luck if you live somewhere that has strong libel laws or free speech protections. Deploy warnings through your lawyer. Stall for time. With time and lack of attention, hopefully, the crazies will become hyperbolic and say so many damning things that you can sue them for something. Make sure your lawyer is trying to obtain details of the internal communications about you, and the things you were accused of by the crazies. Document everything, even phone conversations. Write down names, dates, and times. Take lots of screenshots.
Also, lock down your social media. Archive any important old stuff and wipe the rest. Your thoughts from eight years ago are not that interesting. Purge any public social media regularly. Try to make it look like your accounts got suspended. This will make the crazies happy. Delete your accounts and make new ones under a different name.