Is this forgotten?
Is there a reason?

Some mod's patches looking for stock resources add ModuleB9PartSwitch then and the confusion begins.
Or other weird circumstances.

Related to:

GameData\InterstellarFuelSwitch\PatchManager\ActiveMMPatches\IntegrationLiquidFuel.cfg
GameData\InterstellarFuelSwitch\PatchManager\ActiveMMPatches\IntegrationLiquidFuelOxidizer.cfg
GameData\InterstellarFuelSwitch\PatchManager\ActiveMMPatches\IntegrationOxygen.cfg
GameData\InterstellarFuelSwitch\PatchManager\ActiveMMPatches\IntegrationXenon.cfg

I need to check out how KSP behaves when I prune those 4 configs with PatchManager.

Well the idea is that you only remove the resource in the VAB and it won't modify them when loaded in flight. THat way you won't accidently cause a problem.

Allright, I grasp that.

well I haven't change anything so it s more likely it is related to Module Manager

I didn't mention a change - so I guess it was always like that.

I just don't get the weird concept in creating a fuelSwitcher but NOT removing the stock resource entries.