There is 9-16 extended raid icons are available in DBM,
https://github.com/DeadlyBossMods/DBM-Retail/wiki/%5BAdvanced-Guide%5D-Extended-Raid-Icons-(Obsolete)

As well as in Weakauras
WeakAuras/WeakAuras2#3473

So I wonder if ThreatPlates could support for this

This is no longer possible, Blizzard disabled it. See update in your first link.

Saw that, yeah.