On what particular app are you missing this option?
e.g. Plex for Android TV, HTPC and the iOS apps should already allow that.
https://support.plex.tv/articles/settings-android-tv/
https://support.plex.tv/articles/htpc-settings/
https://support.plex.tv/articles/205671068-settings-plex-for-ios/
Plex Web also has an option to specify the vertical subtitle position (though it’s not specifically mentioned in the support articles (Settings > Plex Web > Player > Subtitle Position) – same for the regular desktop app.
There’s been an existing suggestion asking for similar support in different other platforms – as well as an option to configure font, colors etc. You might want to comment/vote in that thread in order to help us avoid distracting or cannibalizing votes.
