Half of my education in URLs as user interface came from Flickr in the late 2000s.[…]
This was incredible and a breath of fresh air. No redundant www. in front or awkward .php at the end. No parameters with their unpleasant ?&= syntax. No % signs partying with hex codes. When you shared these URLs with others, you didn’t have to retouch or delete anything. When Chrome’s address bar started autocompleting them, you knew exactly where you were going.
[…]
It was a beautiful and predictable scheme. Once you knew how it worked, you could guess other URLs. If I were typing an email or authoring a blog post and I happened to have a link to your photo in Flickr, I could also easily include a link to your Flickr homepage just by editing the URL, without having to jump back to the browser to verify.
— Marcin Wichary (via Michael Tsai)
The advent of Single-Page Applications (through Angular and React) screwed over the beautiful URL design of the late 2000s, of which Flickr is one of the best examples.
Back then, APIs were designed for the public. But the facebookesque progressive siloing of the internet made big companies stop providing public documentation for most APIs, in order to control the clients (where the money is made).