The text in SRM Guide 1.8 is "Does standard equipment consume space for mod slots? No, however removing the mod does not open additional mod slots."
That does not make any sense from a "real life technical perspective" and also not from a role game power balancing perspective.
The German official rules ("Vollgas" which is the translation of "Double Clutch" rule book) state on page 22 (translated) "The standard equipment listed for the vehicles in this and the next chapter counts against the available modification slots of the respective modell (see "Making it yours", p. 118). For example the Toyota Daytripper (body 12) with passenger safety system 2 as standard equipment does not have 12 Chassis modification slots anymore, but only 11".