People who say that are wrong. WotC has indeed stated that the RAI was that using a two handed melee weapon with a caster is not available without feat support (if you use those rules). They have stated that the RAI was that holding a two handed melee weapon required both hands at all times (ranged only needing one to hold, but both to fire).
This is specifically the reason that the quarterstaff is a versatile weapon rather than a two hander, for example.
This is the reason that a holy symbol can be placed on your shield, for example.
Your DM may overrule or interpret differently, but the RAI was that casters can't use two handers inherently, and I'm not the one saying it....WotC said so.
People saying that you can just "rest your sword on your shoulder while you cast" don't understand that actually doing this in a combat situation means that they'd be dead instantly.
except that because of how the Material component exception clause works, Holy symbol on shield doesnt change the fact that you are A: Holding the shield, not the Holy Symbol and B: Dont have any free hands anyway