From the description of the Newcomer Tasks project (see T227728: [EPIC] Growth: Newcomer tasks 1.0) and the initial implementation of the underlying API (see T234426: Newcomer tasks: GrowthExperiments API for template tasks), it appears that the project supersedes the GettingStarted project.
The GettingStarted project was deployed by the old Growth Team in 2013 (?) and active development ceased circa August 2014. Because the team disbanded quickly, however, and the project was never undeployed. Newly registered users still receive the GettingStarted UX to this day.
It's great that an extension not under active development still impacts people's experiences on the Wikipedias and largely a testament to the contributors who have fixed compatibility issues and/or bugs after the extension's authors moved on. Simultaneously, all deployed code is a maintenance burden and the GettingStarted project is no exception. We should discuss whether the Newcomer Tasks project does indeed supersede GettingStarted and whether the latter can be undeployed.
Additional places to undeploy/update references: