The $wgExternalLinkTarget configuration option is mostly site-static, except for a single use in the Installer:
https://codesearch.wmcloud.org/search/?q=setExternalLinkTarget&i=nope&files=&excludeFiles=&repos=
The installer uses an ad-hoc parser anyway (see T317644). It should just reset the site configuration as needed, rather than use a parser option here.
We can start by marking ParserOptions::setExternalLinkTarget() @internal, and deprecate and remove it once the Installer is rewritten to use Parsoid.