Joe Rossignol, MacRumors:

In an internal AppleCare email this week, obtained by MacRumors, Apple said the new serial number format will consist of a randomized alphanumeric string of 8-14 characters that will no longer include manufacturing information or a configuration code. Apple said the serial number format transition is scheduled for “early 2021,” and confirmed that IMEI numbers will not be affected by this change.

Just as predictable file names make hacking and discovery easier on a web site (once you know the name and path of an existing file, you can guess others based on the scheme), randomized serial numbers make guessing/faking a serial number that much harder.