The above posts more or less capture what I have previously stated as my views on backward compatibility. I try not to make changes that break older scripts. I will often even leave non-critical scripting-related bugs in place if they were introduced many years ago. In the case of WM_MCOMMAND, which was added to mIRC v5.9 in 2001, this is the first time anyone has asked for it to be changed. Unfortunately, I cannot change a feature that has been established for so long. I have received many emails over the years from people who enjoy using, and depend on, scripts but have no idea how to update them. It can be such a difficult and frustrating experience for users to ugprade to a new version of mIRC only to find that their scripts are broken. So as a policy, I try to avoid making non-backward compatible changes unless they are necessary. Thanks for your comments everyone.
