Is there a way to add back the "Local Upgrade" feature for OnePlus devices

They don't have exclusive controls.  As I said this has been beaten with a dead horse since the first Android device the G1.  Ironically the earliest Android device got updates when manufacturers wanted to update.  Most of them never updated them at all.  People blamed the carriers including Europe and Asia.  Over time Manufacturers admitted here and there(not all of them) that carriers have little to do with updated. Europe/Asia started to blame OEMs over carriers.  Google who used to blame carriers on behest of US consumers.  Google did a AOSP project with Google and found out carriers around the world on Average because of being blamed for updates which affected network compatibility averaged about a month in additional testing.  What Google and Sony found out majority of the delays came from government body testing from the likes of FCC(or country equivalent) which on average took 3-6 months.  Google then realized carriers were not the delays consumers made it out to be.  They started making Android modular which helped reduce delays in updates.  They release the kernel, and various drivers so OEMs could go through testing through the earlier part of Android development(which Google is making Android 12 for example) to go through testing before Google finalizes the major updates.

So even now US consumers blame carriers and nothing has changed.  People complain why Europe and Asia gets updates quicker without looking at the glaring difference.  Where blame goes. 

As I said earlier if you want quicker updates.  Stop blaming carriers and believe OEMs with their canned responses.  IF Samsung, Google, HTC, Sony, and Motorola can push updates as fast at times or faster in the US then other OEMs can as well.  If OEM's don't want to spend the resources supporting their products that's on them, and not carriers.