The selnolig package, which requires that documents be compiled under LuaLaTeX, may be your friend. It lets users define non-ligation rules via \nolig commands.
The package's main purpose is to set up rules to suppress ligatures selectively. However, it is easy to devise rules that suppress ligatures globally. For instance, to disable the long_s-t ligature globally, one just has to issue the instruction
\nolig{st}{s|t}
in the preamble of the document.
The package also provides a command called \uselig to override the action of a \nolig rule on a case-by-case basis.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{fontspec}
\usepackage{humanist}
\usepackage{selnolig}
\nolig{st}{s|t} % suppress "st" ligature globally
\begin{document}
\hminfamily
Compare: \uselig{straight} versus straight
\end{document}