They're slowly boiling the frog, I've been expecting old. to work for maybe 1-2 more years then it'll also be going away.
Probably in the same way as they're actively removing r/all, at first it just didn't show up in the sidebar on mobile, but you could go to r/all manually by clicking links in the client. Then those stopped working, but r/All (uppercase A) worked. Then that went away. By now I think it's impossible to see r/all at all in the mobile client or the modern website, you can only access it via old.reddit.com.
Probably in the same way as they're actively removing r/all, at first it just didn't show up in the sidebar on mobile, but you could go to r/all manually by clicking links in the client. Then those stopped working, but r/All (uppercase A) worked. Then that went away. By now I think it's impossible to see r/all at all in the mobile client or the modern website, you can only access it via old.reddit.com.