The “Add server” button as always been here on PC, Android and iOS, but has always been completly missing from Xbox, PlayStation, Nintendo Switch and Nintendo Switch 2.
It should be available everywere.
This button is used for adding a server to the server list instead of just being locked to the “official” servers.
Ok after searching a bit I found the original report:
https://report.bugs.mojang.com/servicedesk/customer/portal/6/MCPE-37713
It was closed years ago as “Work as intended”.
If you want to talk about it do it there.