TLDR - I think the lock icon conveys the wrong meaning, is mostly unnecessary and calls too much attention to itself
Lock == locked, which to me means that it likely will be unlocked and can be added to in the future.
Closed means that it has been shut down and there is little likelihood that it will be reopened in the future.
For a forum, traditionally locked means that the moderator has decided to have this pinned to the top of the page and there is nothing you can do about it. Which is not what it means in this case.
The lock in this case is supposed to signify that I can’t add new information to the discussion.
I think if the item is important enough that you need to highlight it at the top, you don’t really need to make people think about whether or not they need to join in on the conversation. The action you are trying to promote is read, not participate.
I suggest a reconsider as to whether or not the lock needs to be displayed in the topic list at all. (I don’t mind the SE [closed]). At least don’t stick it as one of the first dark icons on the row as it draws too much attention to itself for something that will only affect my behaviour one in 1,000,00 times. Think about it, how many times is someone going to see a topic and without reading what’s inside think, I wish I could have participated in that conversation, but it’s closed.
Most people who participated in a closed topic and were invested in it would already be aware that it was shut down.
For the case above where it is an announcement, then the pin functionality suffices.