Highlighted some key words here... There's nothing on your post that leads to the conclusion that this needs to be a rule enforced on all users and all PD bodies... This should be a decision/responsibility of the user (or organization) doing this. Remember that there is other people using FreeCAD, doing different stuff. For example for 3d printing it might make sense to put more than one solid for manufacture at once, even for CNC if two parts are meant to be manufactured from the same piece of raw material it could make sense to model them together. Or what if I'm an amateur modelling and manufacturing my own part? What do I care about datasheets?Or what if it won't ever even be manufactured? In my example above I do not need separate solids for something that will only be used for a boolean op and never manufactured or exported to a datasheet or BOM. There is no reason to force that workflow onto these usecases beyond some silly fanaticism over semantics.
I'm against the single solid rule, not against a feature to obtain a single solid from a body (this exist in Part WB already) or against a feature to be warned when a body is not a single solid (this could even be default on).