Currently bundled extensions are documented like:
{{Bundled|1.38}} {{Extension |status = stable ... }}
that is the information that it's bundled is provided via a separate template (Template:Bundled).
The problem with this is that the extension infobox still contains a prominent Download extension link and the note about the extension being bundled on top of the page is very easy to miss (indeed MediaWiki users might be used to just searching the Download extension link in the infobox). So I think it would be better to set this information via a bundled
parameter, i.e.
{{Extension |status = stable |bundled since = 1.38 ... }}
This could still add the {{Bundled|1.38}} note as usual but it could also additionally show "Bundled since 1.38" right before the Download link in the infobox.