Sorry if this is an old question, I did try to find it but no luck.
Is there a way to set a message placeholder when a thread is closed (rather than moved only) from the system settings or this would be a custom build?
I'm seeing more and more users that are unsure when a thread is open or not (there's no clear indication to whether it's closed or not from a user's perspective).
Any workarounds for this? (if this is not an OotB setting I'm missing)
Currently there is not out of the box functionality on topic page level that would help indicating that the current topic is closed for replies. A visitor to the topic would only be able to tell by the deactivated reply buttons / missing quick reply. Only on the board level you can tell a topic has been closed by the locked icon (in the default styling) shown next to it.
It would be great if you could file a new product enhancement idea so that others can show their support for this improvement.
Looks like @miikka was quicker and just filed "Visual indication of a closed topic on Forum Topic Page" - Please share your support and add details on your scenario in the idea comments.