Having no quote button was a headache for those who wanted to quote from locked topics in a different thread. I ran into that issue many a times before this patch was implemented.

When you reach a locked topic there's a padlock right besides it. When you open the thread, the "reply" icon is also not available and the padlock sign is on the page too. If you somehow missed that, when you click the quote button in a locked thread there's no post option, so that should be a pointer not to writing a response.

I think it's a great feature and was implemented as best as it could be