Thanks! By retrofitting I mean having to have a new protocol with this new opt-in method where some apps will be getting integer scales and go through a transform and some apps will be getting a fractional scale and rendering directly to the output resolution. If this had worked "correctly" from the start the compositors wouldn't even need to know anything about scaling. As far as they knew the scaling metadata could have been an opaque value that they passed from the user config to the clients to figure out. I assume we're stuck forever with all compositors having to understand all this instead of just punting the problem completely to clients.
When you say you supported this for quite some years was there a custom protocol in KWin to allow clients to render directly to the fractionally scaled resolution? ~4 years ago I was frustrated by this when I benchmarked a 2x slowdown from RAW file to the same number of pixels on screen when using fractional scaling and at least in sway there wasn't a way to fix it or much appetite to implement it. It's great to see it is mostly in place now and just needs to be enabled by all the stack.