Apple has just pushed an update to the data files used by XProtect, bringing its version number to 2143 dated 1 April 2021. There doesn’t appear to be any concomitant update to MRT today.
Apple doesn’t release information about what these updates add or change, and now obfuscates the identities of malware detected by XProtect using internal code names.
There appears to be just one change in the XProtect Yara definitions, with the addition of two new hashes to the existing signature for MACOS.1db9cfa (XCSSET variants), which was only added in XProtect 2142 a fortnight ago.
You can check whether this update has been installed by opening System Information via About This Mac, and selecting the Installations item under Software.
A full listing of security data file versions is given by SilentKnight, LockRattler and SystHist for El Capitan, Sierra, High Sierra, Mojave, Catalina and Big Sur, available from their product page. If your Mac has not yet installed this update, you can force an update using SilentKnight, LockRattler, or at the command line.
I have updated the reference pages here which are accessed directly from LockRattler 4.2 and later using its Check blog button.
I maintain lists of the current versions of security data files for Big Sur on this page, Catalina on this page, Mojave on this page, High Sierra on this page, Sierra on this page, and El Capitan on this page.
I am grateful to Phil Stokes at Sentinel Labs for decoding of the obfuscated malware names here.