Finder can show milliseconds - is this new in macOS?

This change happened with the introduction of the APFS file system.

Traditionally on Mac OS X the computer's disk drives were formatted with the HFS+ file system. This file system stores time stamps with a 1 second granularity. If you save a file at 12:04:31.4938, the file system actually only records 12:04:31 and doesn't have any more information for Finder to display.

macOS High Sierra was the first version of the operating system to replace HFS+ with APFS as the default file system. The APFS file system stores time stamps with a nano-second granularity. This means that if you save a file at 12:04:31.4938, the file system actually stores that full time stamp - allowing Finder to actually display that exact time stamp.