https://cabal.readthedocs.io/en/latest/setup-commands.html#cmdoption-runhaskell-Setup.hs-configure-enable-split-objs says
--enable-split-objs
Use the GHC -split-objs feature when building the library. This reduces the final size of the executables that use the library by allowing them to link with only the bits that they use rather than the entire library. The downside is that building the library takes longer and uses considerably more memory.
Modern GHCs no longer recognise -split-objs, one can check it with ghc-9.2 --show-options | grep show-objs. The option has disappeared between GHC 8.8 and 8.10.
I didn't check: it might be not only a documentation issue, but also a Cabal bug, if it tries to pass -split-objs to GHCs, which does not support it.