From the description of the Newcomer Tasks project (see {T227728}) and the initial implementation of the underlying API (see {T234426}), it appears that the project supersedes the GettingStarted project.
The GettingStarted project was deployed by the [[ https://www.mediawiki.org/wiki/Growth/Growth_2014 | 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 [[ https://www.mediawiki.org/wiki/Onboarding_new_Wikipedians#New_experience | 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.