Those are not built-in into Nextcloud. As such they are not “missing.”
What you are pointing out is an app: Link Editor.
As far as I can tell, last version will work in NC30, but has not been tested for NC31.
This suggests that it won’t work for NC31. Hence not showing up.
Go into your Apps and check if you can enable it, if it is not. If it is already enabled than it stands to reason that NC31 broke it. You would have gotten a list of apps that could break before you updated, this app would have been listed there.
You will have to wait until the Dev updates it, or you can go into the App folder and change the range of versions it works on. Keep in mind that if the app is actually broken, doing this will not make a difference, or could make things worse.
You can search online for ways on how to do it, if you want to try. Otherwise, wait until it gets updated.
Cheers.