Ah, I've had that problem on one of my accounts. Basically it's when your comment P-score sinks too low due to inconvenient truths. And you're right, even something as basic as "Hi" or "Cool" would get hidden.

It took about two months before any comment would show up again.

I read one article some time ago that said it takes 48 hours with zero activity on the account before your account is no longer globally shadowbanned. That only worked for one account, but for the other required much longer before it could post again. I let it sit for a couple of months with zero activity and then tried it again making a seemingly benign positive comment and it showed up again when in incognito mode.

I imagine the lower the account P-score the longer it takes before the account is no longer shadowbanned. Sort of like waiting for the social credit score to reset to default.