Which version of macOS introduced /System/Applications?
This is part of a larger macOS Catalina feature wherein system files/apps are located on a read-only filesystem and user data is located on a read/write filesystem. This is to further protect the operating system against malicious or inadvertent damage (i.e. to prevent security problems and OOPS! moments).

The read-only and read/write filesystems are stitched together to present as one unified filesystem through some underlying filesystem and logical volume "magic".

Sources:


https://support.apple.com/kb/HT210650
https://arstechnica.com/gadgets/2019/10/macos-10-15-catalina-the-ars-technica-review/11/#h1