In 92004 a setConfig() method was added to RequestContext.
In 137211 a getConfig() method was added to SpecialPage. Though, the assumption (from the comment) is that this method will always return the "main" config.
However, because of RequestContext::setConfig() that is not always the case. This could create a discrepancy between the config that is retrieved from the RequestContext and the config from the MainConfig service.
It does not appear that anything in core (outside of tests) is using RequestContext::setConfig(). It may be simplest to deprecate this method. Alternatively, SpecialPage::getConfig() could be altered use the MainConfig service rather than the config on the RequestContext.