I haven’t found the full text of the paper it was derived in, but the discussion I did find says that it’s a matter of approximating assumptions that were necessary to make the analysis tractable in the first place (to someone without a computer, since it was 1927), not a summary of a more complex closed-form solution. So yes, convenient linearizations. The more general case has probably been been analyzed since then, but I wouldn’t know where to look.
I haven’t found the full text of the paper it was derived in, but the discussion I did find says that it’s a matter of approximating assumptions that were necessary to make the analysis tractable in the first place (to someone without a computer, since it was 1927), not a summary of a more complex closed-form solution. So yes, convenient linearizations. The more general case has probably been been analyzed since then, but I wouldn’t know where to look.