Here's a few screenshots to show what I'm talking about:
The mIRC in that screenshot is a freshly-installed one, version 7.29, with no scripts installed aside from the ones that come with it by default.
It turns out I was incorrect about X-Chat supporting messages of arbitrary length, but my point still stands. IRSSI shows the original message in its entirety, and mIRC truncated the message coming from the server to 983 characters.
I know this won't make existing IRC servers support longer messages unless the hosts decide to implement support, but it would be beneficial to those who do use servers that explicitly support messages longer than 512.
And it would encourage existing server owners to make their server support longer-than-512 messages, because more IRC users can view them.