Reddit (the company) is a wretched hive of scum an villany, where this sort of stuff seems almost regular. Perhaps it wasn't always so: I don't know.
But I can't think of any other forum that would put up with this. If an owner or mod did this on the *chans, the chan in question would be abandoned within the week.
As for HN, dang does as he sees fit, as do the admins: they'll split your thread, redact your post (and if they doesn't, the software will if it gets flagged enough, IIRC), and (or so I heard) even shadowban you of you're bad enough. However, they're mostly right, and not corrupt. And they wouldn't pull crap like this. Blocking a comment that talks about how HN is just a massive circlejerk is one thing: actually altering it to say something else is quite another.
I suppose it might be common on forums run by egotistical gits, but come on, you're heading a company running a large forum/news aggregator. You can do better than an egomaniac with a website.