If you can't use something, you could argue that it causes "harm" to your well being or job prospects or something.
But I agree that "harmful" should probably stick to more significant, mostly physical effects.
But, of course, here we are on HN discussing it, so it had its desired effect...
I can imagine a person having more of their life exist on Discord and if it suddenly became unusable for them due to these changes, it would be a catastrophe. Some people literally can't pay rent without Discord, since they use it to prepare content for work or to collaborate with their team.
The product itself is not harmful, just inaccessible, but describing the changes as harmful seems right.
I have to use Discord every day for work, so if this redesign was interfering with that I'd be pissed. (I pay for a third-party Discord client for this and other reasons)
Nobody is inflicting any harm by choosing a bad color palette.
https://www.fdic.gov/resources/supervision-and-examinations/...
I don't have terribly good eyes, but I'm happy to say I don't feel like I've suffured permanent eye damage from the discord UI :]
When OP says incredibly harmful and catastrophic I think they're just describing their feelings and how important it is to them.