From the game's encyclopedia
Sōryō is a type of Feudal Government unique to Japan.
Sōryō represents local lords who have discarded or supplanted the reach of the central Ritsuryō government and rule provinces as their own fiefs.
Sōryō Characters do not use Influence, but Noble Family Heads retain their Family Manor and House Aspiration.
Though they cannot be elected Ruling Regent, Sōryō Rulers may take power via the Install Ruling Regent Faction, and use the Formalize Shōgunate Decision.
Sōryō Ruling Regents govern the Realm on behalf of the Japanese Emperor, inherit by Primogeniture Succession, and may be overthrown by Faction demand.
Sōryō House Heads may create and join House Blocs within the Realm.
Japanese Governments have a lower Domain Limit than others, which may be increased by upgrading the Family Manor.
Sōryō refers to the early stages of Japanese feudalization beginning in the Heian-period, increasingly formalized under the Kamakura-period's first Shōgunate.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Sōryō Government is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "Sōryō is a type of Feudal Government unique to Japan." It is part of Government. The game also links it as Sōryō. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.