There are plenty of modern websites using a modal or login window approach without relying on legacy code. Just to clarify, are you saying that the seamless login overlay, where the login box appears over the Discourse forum in the background, can only be maintained using legacy code?
Legacy has its place. For example, https://discussion.fedoraproject.org, one of the most active Discourse forums that I know of, still uses the dropdown nav bar instead of the left-hand sidebar. My view is that changes should be necessary, subtle, and ideally requested or clearly beneficial to end users.
To be honest, the new login approach feels more like a 1999 vBulletin interstitial login page than an improvement. From a user experience standpoint, I don’t see the benefit. (IMO)
EDIT
Thanks, I did spend some time looking for a thread like this.
Another note is that some forums niche will be more desktop than others. My forum (and WordPress blog) both see less than 25% mobile traffic. Currently, 22.74% last 30 days. So I would prefer to prioritize the 3/4 than the 1/4. Maybe this full-screen login can be mobile only.
Hold that thought, I just tested on mobile, there’s no background. It looks fine on mobile, even with this unchecked:
I’ll edit this post via mobile and include a screenshot. But I would prefer personally if this change can be for mobile only. If it’s even necessary.
mobile edit
Login looks already good:
Oh no, here’s @Jagster chiming in. 
lol