The memory wiping is presented in-game as a way to guarantee safety for the mission, not a "we mindwipe all followers".
Even with my knowledge of Shar, the way she is presented in the interactions with Shadowheart is not very evil. Just a secretive goddess that's being very paranoid.
So for someone new to the Forgotten Realms, Shar's backstory isn't really layed out. It requires meta knowledge.
As I assume Larian wants more people than d&d fans to fall in love with bg3, some basic background of Shar would benefit the player base before Shadowhearts reveal.
