the ace arsenal from main menu will try to load last loadout and throws that error often with changing modsets
https://github.com/acemod/ACE3/blob/master/addons/arsenal/missions/Arsenal.VR/initPlayerLocal.sqf#L6-L11

I think that might be the root problem, otherwise I'm not sure how bad backpacks are getting into the list data

the ace arsenal from main menu will try to load last loadout and throws that error often with changing modsets https://github.com/acemod/ACE3/blob/master/addons/arsenal/missions/Arsenal.VR/initPlayerLocal.sqf#L6-L11

I think that might be the root problem, otherwise I'm not sure how bad backpacks are getting into the list data

If it's only that, we can easily address there.

some private mods use event handlers on list of backpacks after loading list in arsenal ( appending ID to classname for some reason ), so each time they refresh list, there is an error that classname does not exist ( mainly because of setUnitLoadout internal restrictions). Error is omited on every other classname, but backpacks are special, they are type of Vehicle

some private mods use event handlers on list of backpacks after loading list in arsenal ( appending ID to classname for some reason ), so each time they refresh list, there is an error that classname does not exist

This is, in my opinion, not a good reason to add it. Invalid items shouldn't be added to the arsenal to begin with. This should be fixed by the offending mod makers, especially if they are manually adding invalid items.

some private mods use event handlers on list of backpacks after loading list in arsenal ( appending ID to classname for some reason ), so each time they refresh list, there is an error that classname does not exist

This is, in my opinion, not a good reason to add it. Invalid items shouldn't be added to the arsenal to begin with. This should be fixed by the offending mod makers, especially if they are manually adding invalid items.

I agree with this statement, but it is not a main reason, but one of many. Private mods are used, yet they are abandoned and no source code provided for handover. Meaning no other way to address issue other than adding NON-breaking functionality to ACE arsenal.