Lets consider some facts:
-Within days after the update which caused the problem (over 5 years ago), it was confirmed that the Steam client was causing harm to users with either or both of 2 medical conditions, and has since been found to be affecting several other photosensitive conditions (the first 2 being the ones which come with a risk of hospitalisation).
-Multiple users with no prior history of any computer software behaving as a trigger for our conditions have found that the Steam client appears to be a singular exception to that since this update.
-I've directly tested it and confirmed that the Steam client itself was causing issues for me personally, and ruled out the possibility of the trigger being any other software I was running through or alongside the client.
-It's been over 4 years since I stopped trying to use the Steam client, and in that time, I've regained access to about 90% of the games I had played on Steam while it was causing problems, and none of them has caused any issues when played outside of Steam. The one game I had been playing at the time which I haven't regained personal access to, I've watched a friend play on a different platform, and also had no issues while doing so.
-In more than 30 years of computer use, I've had no instances of any game being found to act as a migraine trigger for me, nor any other computer software, except for the Steam client, in spite of playing easily 10x as many games as I ever had on my (functionally stolen) Steam account, which was worth over $3000 at the time I was forced to abandon it.
-I picked the demo of this game up before it had a Steam page, and have played well over 50 hours of it (probably more than 100, though I haven't kept accurate track). I've never tried running the game through Steam, and I've never had it trigger a migraine in that time.
-Just in the past year, I have hundreds of hours in games more flashy than this one, also without any issues.
Given that there are multiple people other than me who have confirmed they've had similar issues, and I've been involved with tests which confirmed it to be a problem for me, I think putting the blame on Steam is pretty solid. Even without that, having 50+ hours of playtime without it ever causing problems is a pretty clear indication that this game has never been the cause of my problems.
From various users I've seen discussing the issues, there's been more than half a dozen separate specific problems identified, but also a few instances (like at least 1 of my issues) where the details are harder to pin down in a definite way.
The most obvious and easily tested one requires a relatively large library, when it gets into the range of 50+ (depending on screen resolution and zoom settings), scrolling through your library can cause very obvious visible flickering effects as the app re-loads already-loaded assets instead of just keeping them in memory while off-screen. This behaviour in several parts of the app is also responsible for memory leaks which can in some instances result in entire system crashes if you leave your PC running with Steam in the background for long enough. That's extremely rare, but the app eating upwards of 4GB of RAM is fairly common on PCs which are left to run for longer periods of time without restarting.
One of the (very) few issues they actually did fix was that many game icons were animated, and there wasn't any way to disable those animations. So they added the option. Except not really, because when they first added the option, it didn't work. So they "fixed" it. And it only took about 6 months of them "fixing" it repeatedly before it ended up actually getting fixed.