But I don't see any particular reason for where exactly the line between the two should be - why wouldn't it be that I could make anything I shared with you ephemeral, a-la Snapchat? And going back to email, it was actually MS Exchange who have supported the option to "Recall this message" [0] for many years, while gmail decided to not implement anything like that.
[0] https://support.microsoft.com/en-us/office/recall-or-replace...
My thoughts:
If it's in my inbox (whether on my physical phone, or hosted for me by a service provider), I own it and I get to control it regardless who sent it / how it got there (as long as other legal pre-requisites are met, i.e. not child porn etc)
If it's in my outbox, and I want to delete my account, THOSE instances of those artifacts should be removed, whether from my physical phone or by the service provider hosting them for me. If I've sent them to others, fair game, they own THOSE instances; but I expect the service provider to remove, upon my request, the artifact instances I solely own.
In this case, Discord is a very centralised service. They store all of the messages centrally, so can easily comply with the request.
Other services like email may not be able to do that, so they could use that as a defence when asked why they aren't deleting all data.
The question is whether this law is closer to fuck you than it is reasonable regulation of bad behavior.