The new pages like above are React apps.
The old pages like below:
https://www.reddit.com/user/HeinieKaboobler
Are server side rendered.
It's not a popular change. Mostly because of people feeling it'd pull focus from subreddits to profiles, but also some complaints about the performance loss and lower density: https://www.reddit.com/r/announcements/comments/60p3n1/tldr_...
If they made a technical post about them I haven't seen it, but there was an announcement for them as a feature.
I just know that my react-devtools extension and the redux state inspector both lit up when I went on that page.
https://www.reddit.com/user/spez
Aren't those just static html pages rendered serverside?
Is there a blog post about them or something?