Peter Selinger’s fitch.sty package is a popular way to produce nicely formatted Fitch-style natural deduction proofs (e.g., in the free/open logic textbooks of the forall x family). It is now maintained again and a new version is in preparation. The new version will make customization (of dimensions, formatting of justifications, shorthand macros for rule names) more easy. It is not 100% backwards-compatible. In particular, the internals have changed so that if you made your own modifications to the code, these modifications may no longer work.
You can find the beta version of the new package on Github. You may wish to look at the PDF of the current manual. I’d appreciate it if people would test it out to make sure it works as expected. If something is broken (or you’d like to request a feature), please file an issue.
Richard Zach
@openlogic @openlogic Note: it is also on CTAN https://ctan.org/pkg/fitch #TexLaTeX