Lead Encounter Designer Ion "Watcher" Hazzikostas has taken to the forums to reassure players on post-squish soloing. Despite Blizzard's repeated assertions that the squish will do nothing to affect soloing, the message is going unheard by many, so Ion has provided some clarity on just how the downscaling will work for old raids and the like.
That's great! But I could have sworn solo raiding was frowned upon by Blizzard. Not sure they'll be too worried about keeping it a possibility. Hopefully though.
There's nothing at all wrong with going back and doing legacy raids and other content. The achievement system, cosmetic rewards and titles, and most recently transmogrification all serve to reward that playstyle. There's also a lot to be said for just being able to revisit old locations, for the sake of nostalgia or getting to experience them for the first time, without needing to find a like-minded group. As Rygarius noted, we're committed to making sure that we don't disrupt players' ability to engage in that gameplay.
Without getting into the math, our goal is to make sure that if Kael'thas's Pyroblast does damage equal to 10% of your maximum health today, it will take off no more than 10% of your health post-squish. And if you kill Onyxia in 30 seconds today, you'll be able to kill Onyxia in 30 seconds post-squish.
Ion's words make perfect sense. For some reason some players seem to feel like the soloing of old raids isn't something Blizzard wishes to support, cares about, or even likes. But they've introduced ever more incentives to play the game this way -- as Ion notes -- with transmog, achieves, titles, and more recently with the addition of pet drops and changes to encounter design to make soloing more straightforward. That they would remove that gameplay with the squish is highly unlikely.
But some more news on just how the scaling will work is welcome. The percentage-based idea of damage makes it easy to understand how the squish will work, and how your experience soloing old content won't change. The numbers that cloud your screen and drop your framerate when dealing with trash will simply be a few digits shorter.