A source code requirement would not meaningfully impede software patenting. The teeth of a patent are its claims, not its enablement. The premise of this post seems to be that a patenting entity would be required to disclose its actual commercial source code, but of course, were source code required, even given the best mode requirement, it would still be horseshit source code.
The best-mode requirement was pretty much neutered as part of the America Invents Act, because:
1. USPTO patent examiners normally don't inquire about best mode unless there's a pretty-obvious failure; and
2. in litigation about an issued patent, "failure to disclose the best mode shall not be a basis on which any claim of a patent may be canceled or held invalid or otherwise unenforceable ...." (35 U.S.C. § 282(b)(3)(A), at https://www.uspto.gov/web/offices/pac/mpep/mpep-9015-appx-l....)
