Bitcoin is 100% immutable UNLESS it's changed.
Well, what isn't? :-)
The thing is, forks happen all the time in Bitcoin, when two miners find a block at roughly the same time. In most cases these forks are resolved pretty quickly. Sometimes it takes several hours though. The built-in rule is "longest chain wins".
From a technical POV there is no difference between these random forks and a deliberate decision by miners. Calling it an attack is therefore questionable, IMO.
Yes, but attacks are not technical, but political and economical. The intent is different and that's key in this conversation.
When miners collude to change transactions or prevent transactions, you've lost your immutability and censorship resistance. That's different than two miners finding a block simultaneously and then it becoming a game of longest chain wins.
What I'm trying to say is that BTC is immutable in the same sense that ETH was immutable until the DAO hack. In other words - not at all.