So here is what DJI has said before about OTG:
The primary reason OTG isn't supported is that the RC is NOT MFi certified (Apple), meaning it doesn't have their authentication chip in it. Call it cost savings or whatever. Therefore, Apple might kill wired access at any time (of note, the iOS 11 update killed all my 3rd party cables that I have been using for 3 years, even though they claimed to be MFi, only the original short Apple cable works now).
The OTG connection is necessary for the DJI goggles to work (they don't have wifi), and optionally for CrystalSky. Both of these SUPPORTED devices run Android. Therefore, the OTG connection must remain active.
That said, DJI doesn't want to support OTG as a feature on all devices with all that testing, instead focussing on just their two products. If they really wanted to, they could build in authentication into GO4 that if it detects a Spark RC it won't connect by USB, and that would be the end of it. But at this point, the cat is out of the bag on OTG, so it is better for customer satisfaction to just not officially support a feature the device never officially had than to kill it.
Reality is, OTG is there, it works, and likely will work until the end of time, but DJI will never officially say so.