Seriously, what the fuck? I thought it was some kind of user interface glitch so I looked it up. Nope, they're A/B testing this shit.
Found this brilliant statement by Youtube on the matter:
Now note that we are deprecating one sorting option and that is by oldest video at the channel level. But don’t panic, you can still view the oldest video of a channel, by scrolling through it’s content’
Oh, yeah, just scroll to the bottom. Silly me, I guess I'm just a stupid idiot for not realizing the option was redundant since I can simply take the time to scroll through potentially thousands of videos.
Why are silicon valley tech people such freaks?
My guess is that it was requested by creators who feel like their old stuff isn't up to standards.
Also a bit off topic but in my opinion the browser extention PocketTube is a must have for anyone who uses a lot of youtube. It adds a ton of useful features that it’s honestly embarrassing that google can't deliver something similar.
Maybe it was the advertisers and marketers that decided that the older stuff isn't up to standards. I really doubt YT cares what creators think any more than any other businesses or politicians; just enough to keep the status quo.