This task suggests the goal to dramatically reduce the use of global context across both core and extension code.
It could be closed once RequestContext::getMain (or analogous pattern) once there is

• a precise list of scenarios where calling RequestContext::getMain is truly appropriate,
• code is adjusted to only call RequestContext::getMain in precisely these cases.

This would include many complex injections, hook signature adjustments, and potentially deprecations of entire code paths.
This is a very long-term goal, which could well stretch across more than a decade.
Using RequestContext is still preferable if the alternative is calling globals like $wgLang, $wgRequest, $wgOut etc.