The GTA: Underground team left the project
Global modification GTA: Underground was among those which Take-Two demanded to be removed from moddb.com, and taking into account that oppression of mods is still not stopped, the team made the decision to completely stop development.
This was announced, including on GTAForums.com… Developers are showing interest in creating their own game, but so far I can neither confirm nor deny such a possibility. They promise to keep in touch, but for now they have presented a farewell video dedicated to the project and made in the style of the closing credits of GTA.
GTA: Underground Is a huge mod based on GTA: San Andreas, which includes maps from GTA 3, GTA: Vice City, Bully, Manhunt and Manhunt 2, and also combines a whole bunch of content from these games. The development was carried out for six years – since 2015. The latest version of the modification is 4.2.
Source: gta.com.ua

GTA 6 Is the Most Preordered Video Game in Console History
GTA 6 Boxes Have No Disc or Map, but Include a Poster and New Screenshots
GTA 6 Preload Now Available
Zelnick on Completing GTA 6, Its Launch, and PC and Nintendo Switch 2 Ports
Take-Two Financial Report: GTA 5 and RDR 2 Continue to Sell Steadily
GTA 6 Music Festival to Be Held in Miami
GTA 6 Has More Than 50 Achievements
IGN’s GTA 6 Preview Covers Naked Jason and NPC Confrontations