This was brought up by an extension author I was talking to some time back, but I forgot to report this.
It's known the old preferences system is deprecated and will be removed (I already have a PR up to remove it). The PreferencesManager.convertPreferences() method, used to convert the preferences to the new system, will obviously be removed at the same time. However, as the author mentioned to me, the method throws no error saying it will be removed in the future. In fact, it works silently, and the deprecation warnings only fire if the old get/set methods are directly accessed, not when preferences are converted.
The author thought convertPreferences() should be throwing a deprecation warning to alert that it too will soon go away, and with a select few extensions still using the old/conversion system, I think it may be a good idea to quickly add one so it will go off in the v1.2 release, which will could allow us to remove the old system as early as 1.3 or 1.4.
(Side note: If you are reading this and are the extension author I spoke to, please speak up. I have completely forgot who I spoke to 😢).