Wrong tool for the job
Repost from 2018
This category is for an answers that use the worst possible language to accomplish a task, while still making an effort to optimize the score. For example, writing a non-trivial program while using an extremely minimalist language like 7, or an inconvenient language like lost, or even a normal language that's missing some crucial capability like internet connectivity or image processing.
Answers for this category should consider both the difficulty of the task and the unsuitable-ness of the language.
tjjfvi's 25 answers in AOCG2021 in TypeScript Types
Nomination by alephalpha
It's well-known that TypeScript's type system is Turing complete, but writing nontrivial programs in TypeScript Types is not easy. tjjfvi solved all 25 challenges in AOCG2021 using TypeScript Types, which includes writing an interpreter and solving word problems of Hyperbolic Coxeter groups.
user197974's Gravity Simulator in Marbelous
Marbelous is a famous turing incomplete 2d language which is just unsuitable for most simple tasks. The author of this answer has gone so far as to write an entire interpreter for Marbelous themselves, along with simulating a rotating tape within its constraints to make a truly impressive answer.
caird coinheringaahing's parsing HTML with Jelly
Self-nomination
What's a worse idea than parsing HTML with regex? Parsing HTML with Jelly
Jelly almost famously has very little string handling capacity, is awful at parsing challenges and, being a golfing language, isn't exactly optimised for web-based ones either. Which is why it's very surprising that Jelly is the winning answer here, in a challenge about parsing the HTML content of a webpage. I think the quote from my answer just about sums it up nicely how bad Jelly is for this challenge.
Aaroneous Miller's's Whitespace interpreter in Vim
Nominated by DLosc
Vim is a text editor. Sure, it's got tools that make it far more capable than your average text editor, but still--it's a text editor. When you think Vim golf, you think of transforming some input into some output, usually string manipulation, maybe a little math.
Aaroneous Miller used Vim to write an interpreter for a programming language. (Actually, this happened twice (!), but the BF interpreter was already nominated for another category, so I'm nominating the Whitespace interpreter here.) The amount of patience required to implement a language with control flow, a stack and a heap, and two dozen commands in Vim is amazing.
