WD has finally released an updated source code of MCH. It’s now based on 6.6.1-123 up from a 4.x from 2017. see: https://support-en.wd.com/app/products/product-detail/p/135#WD_downloads
I’m no developer but it allows to check the software version they are using inside MCH.
Samba is at version 4.4.0 from march 2016 and discontinued in September 2017 (see: https://wiki.samba.org/index.php/Samba_Release_Planning)
It does not support Time Machine over SMB which started in 4.8.0 in March 2018. Samba is now at 4.11
On Time Machine matter, it runs the latest version of Netatalk (AFP client): 3.1.12 of December 2018.