Here is my best guess on why this plugin has yet to be updated...
First and foremost, Oxwall is an open source software. This means that contributions made by the community helps oxwall grow - those who complain over an issue for an extended period of time do not help. If you did any reading in this thread, you will note that there IS a fix to the issues you are encountering for this plugin.
http://www.oxwall.org/forum/topic/29195?page=3#post-127021 - (Broken down version)
http://www.oxwall.org/forum/topic/29195?page=4#post-128274 - (May be complicated for some users if you do not know what you are doing)
Sure, this means that you yourself have to take responsibility in fixing the issue, but that is pretty much what you signed for when using open source software.
Secondly, since this messages plugin is FREE, it too is considered open source. And since I have been an oxwall user for quite sometime now, I can attest that some issues cannot be fixed by the devs as they are unable to replicate and/or reproduce the issue.
Thirdly, when there is an update to the actual platform, that's when these freebies get updated as well, (e.g.: messages plugin, blogs, groups, events, etc.) - essentially all the FREE plugins that come with the platform. In total I think there are 19 plugins that come with the platform.
So, if you do not want to take matters in your own hands and trust the community for help. Please be patient.
To re-iterate:
1. Oxwall is open source, this includes the messages plugin.
2. Read the entire thread - there are actually 2 solutions to this issue (both do the same thing, 1 way is broken down for you, the other way is/or may be a bit more complicated for you if you do not know what you are doing. However, both accomplish the same result - a fix).
3. If you do not want to take matters in your own hands, please be patient. I'm sure this plugin will need to be updated once a new version of oxwall comes out (this may still be awhile, so you may need to resort to #2).