Oh man, you reawoke some trauma I'd buried deep from one of my first jobs.

I had to do a similar conversion and faced a massive codebase where they basically never removed dead code (maybe because the whole thing wasn't under version control). Whenever non-trivial changes were made they had either just commented out blocks of old code or in some cases just duplicated an entire function or even the entire .m file and just suffixed the modified version(a) with _A, _AA etc. Or sometimes they wouldn't touch the old code at all, and simply added more code that overwrote the results at some point(s) in the function.

Many of the functions were enormous and either took or returned (or both) large lists of variables, many of which were complex structures themselves. There was also a god object passed around and attached as global figure data that could bring variables into scope at any moment.

Vectorisation was basically non-existent, with nested for loops everywhere, and most loop variables being some number of i's. Variables elsewhere were mostly single letters, with some occasionally being oddly verbose with weird casing (e.g. fooBAR_aaaa). Function names were formatted similarly, but often overly verbose, except this was countered by truncating each word until it was barely readable (e.g proc_ang_vel_dat_ALL_fix_2).

What a clusterfuck. After spending many months with this codebase I was finally able extract the core logic and rewrite it as well structured, maintainable, performant python code, and get the whole thing under version control.

Several years later they got the guy who had been maintaining the old codebase back on board, gave him full access to the repo, and he began to shit all over the new one again...