It's not necessary.
First of all, even WE recommend that some
users continue to use 6.35. See issues regarding codepage support
. Breaking these versions would contradict a lot of the advice we're already giving.
Secondly, I don't know of any security issues with 6.35. Where is this claim coming from?
Finally, as was mentioned, upgrading is the user's choice. It's not mIRC's place to force people to do anything they don't want to. mIRC already informs you of new versions, I believe that's sufficient. Education is more important than hand holding.