What does the board think? (Assuming readers know the historical meaning of mistress..)
And if mistress is outdated, what is the current concept/ term?

Lots of them are discussed on the Erotic Highway board.

Just what is the "historical" meaning of mistress?

go to any one of numerous sugar baby sites   like Seeking or  whatsyourprice     there are plenty who wish to have that life role.

At the core of it are two things.. compensation and companionship.  As long as these two are involved, it will be never outdated.  Yes, the packing has changed, and no one uses that word anymore.. Most people call them Sugar Babies now..

for guys that are so rich and/or famous that women will fuck them for free on spec.  For regular Joes, as others have said, SB's are the mistresses in modern culture.