That has always been the case, yes, though I'm not sure what you mean by "illegal" content. There is only a small overlap between NSFW and illegal content, and the NSFW filter has never been concerned with, uh, violating photograph copyright or something.
You don't have to take my word for it, just check it yourself, although it seems that this week, they renamed the NSFW setting to "Age-Restricted Channel" (in preparation for this change, no doubt). The verification-related portion of the behavior I described was implemented for the UK months ago.
The description still contains: "Age-restricted channels are exempt from the explicit content filter."
EDIT: IANAL (or american) but if Discord was policing content for legality rather than age-appropriateness, wouldn't they lose DMCA Safe Harbor protections?
> The description still contains: "Age-restricted channels are exempt from the explicit content filter."
Wait! This does not mean they do not scan it. What I understand from that statement is that they filter explicit content, as in they prevent it from appearing on the user's screen.
When you enable the "NSFW" mode, you tell Discord "it's okay, don't filter out anything". But Discord probably still scans everything.
So that makes sense to me: if you don't validate your age, then Discord will not allow you to join channels that disable the "adult" filtering. I can personally live without adult content on Discord...