There’s a difference between simplifying a message and writing at a lower reading comprehension level. I think a lot of accidental incomprehension might just be caused by the reader not being very good at reading English.
In my country (and I think the whole EU), government agencies have to write at a B1 level to make sure official publications and letters are accessible to all citizens. I think that’s a good rule of thumb for online conversations as well. (However, writing pleasant prose at B1 level is a whole different beast. Generally, they’re not very good at it.)