Multi-user OSes are nothing special. Normally one user cannot access the files of another user. Installed programs are shared. User's are running in a sandbox and they're isolated from other users.
Isolated programs are not that common. It forbids one program to access the files of another program.
Imagine you are using a web browser and you're also storing passwords for websites (so you do not have to enter them again). The passwords are stored obfuscated but unencrypted in a local file in your home directory. Now when you run a game (downloaded from the internet) it can normally read all your passwords. (AFAIK this is still the case for modern browsers today if you are not using a master password.)
On smartphones (Android + iOS) this is different (and in my opinion better). Apps cannot read the files and settings of other apps. Because apps are isolated it's also possible to restrict other permissions like camera access or access to your geographical location.
Which was the first OS having both user and program isolation? At least programs must not have been able to read files of other programs. Also note it must have been possible without modifying the OS, changing configuration files or installing tools.
Also note that the answer "every multi-user OS can be used to isolate programs" does not count. Because to be a valid answer the OS has to be used in a normal way. Manually creating a user for each program on Windows or Debian means the OS is not used in a normal way.
To meet the condition for being a multi-user OS it must be possible for multiple real persons to use one single device (every person has its own user). Also each user must have his own settings for each program (because the settings are stored in the user's home directory for example).
I am looking for all kind of OSes (not only desktop).