We always used Gtk.MessageDialog for user error.
There was never the intention that such error message being copy/pasted.
The fact that it is possible in the web client is just a consequence of the platform.
We are not going to implement our own message dialog just for that.
The fact that something has been done for ages does not mean it should not be changed.
Reading your link I see that:
It’s simply a convenience widget; you could construct the equivalent of GtkMessageDialog from GtkDialog without too much effort, but GtkMessageDialog saves typing.
I do not see any valid reason to prevent an user to avoid copying an error message. On the other hand I see some valid use cases. So I do not understand why you say we are not going to implement it. Could you share your rationals? Thanks!