MessengerAPI, as found in msmsg.exe\3, still works. Messenger, as found in msmsg.exe, doesn't.
I haven't actually got MSN 5, so I can't confirm it, but someone tested it for me, and the program was able to create the first object but not the second.
To confirm if this API is available on your copy of MSN (5.0 or whatever) simply download Msg++ 0.1 (
http://www.nmitchell.co.uk/programs/msg++/) and run it, if it doesn't crash straight away then it works.
If this is the case when someone installs 5.0 directly, instead of upgrading, or uses MSN 8.0, or uses XP, or whatever, I don't know, but it works on at least one copy of 5.0.
It has get status/signin name etc. The ones it is lacking from Messenger are
set signin name, who is not on your contact list, and a few others.