Would you be so lucky if your legacy code was written in a well-known language. At a previous employer we had a big pile code in a “macro” language. It could interface with the main C++ code base. It was similar to assembly language in that it had no loops (just gotos) and register-like local variables (no custom names, just L0, L1, L2,…). The semantics were weird, something like unexpected pass-value-value versus pass-by-reference behavior when calling functions.

One customer required lengthy qualification processes when changing the software. But “configuration changes” had a lower bar and somehow these “macros” counted as config. So eventually all the interesting business logic and RPC processing end up in a giant macro file.

Sounds similar to, but not as bletcherous as, BANCStar, a famed "esolang" that was really the output of a form generation tool used in the finance industry in the 90s. Some engineers figured out that the form generator's output was actually expressed in a Turing-complete, though very arcane, notation replete with opcodes and operands, reverse-engineered it, and started working directly in this language without using the tool to do more sophisticated things than the tool enabled on its own. The boss man probably said "Great! When can we put this into production?" and it's been downhill since.

https://esolangs.org/wiki/BANCStar

https://github.com/jloughry/BANCStar

Relatedly, a long time ago I discovered that Zenographics Mirage—a vector graphics/illustration program originally designed to run on like, VAXen, with both a text terminal and a graphics terminal with a digitizing tablet (similar to this setup seen on Reading Rainbow: https://www.youtube.com/watch?v=b_zYaIxb6dY&t=965s), but later ported to PCs—ran on a sort of bytecode. There were a number of sample scripts shipped with the package that allowed you to automate things in Mirage's command language. Some of them had statements like DO 62,2,32472,32476,0 and comments that read "Don't worry about this, this is just Mirage assembly language." Intrigued, I discovered in the manual a feature you could enable called Op Code Monitor that flashed similar numbers whenever you entered a command. It was mentioned but not documented in detail, nor what the numbers meant, but from that and the scripts I could make some pretty good guesses. I figured out how to make Mirage prompt for a point and store it in a register; and with that I made a command to draw a rectangle that could be rotated. A rectangle in Mirage was defined by its corner points, so when you attempted to rotate it it just rotated the corner points and drew an axis-aligned rectangle with the new corner points. My command accepted two corner points and drew a polyline, so that when you rotated it, the whole rectangle rotated.