I highly doubt that they created BG3's (simultaneously both too restrictive and not restrictive enough) weapon slot system with the intention of restricting shield usage specifically.
I think the most likely scenario was that Larian didn’t want players to have to manually switch between weapons sets before being able to use abilities in the hot bar. As it is, you can freely use melee/ranged skills, no matter what weapon set your character appears to hold. Constant shield bonus, is just a side effect of BG3 not really having switchable weapon slots - our characters have both melee and ranged equipped at all times.
With how the game is set up, allowing players to equip two melee or two ranged weapons would create new issues - as the game is set up there no way to specify with what weapon we want to attack. We have menacing strike on our hotbar - but with which weapon, if we have two melee weapon slots? In my book it’s another black mark against already unnecessarily unwieldy hotbar design.
As such we have one melee and one ranged - that’s also why there is both melee and ranged sneak attack - create one, and you cannot specify with which weapon you want to attack.
Needless to say I don’t like this approach at all. I think it is attempting to solve an issue that never existed, while creating host of new problems and unnecessarily limitations. Damage types tend to be a big consideration for melee classes in later levels - being forcefully limited to one melee/ranged weapon slot might get annoying later on.
