%!TEX engine = LuaLaTeX

\documentclass{article}
%\usepackage{mathtools}
%\usepackage{lualatex-math}
\usepackage{unicode-math}
\usepackage{xparse}


\makeatletter
% Small amount of expl3 code for the DRY principle
\ExplSyntaxOn

\cs_new_protected:Npn \michael__mathchoice:nnnn #1#2#3#4
  { \mathchoice {#1} {#2} {#3} {#4} }

\cs_generate_variant:Nn \michael__mathchoice:nnnn { oooo }

\cs_new_protected:Npn \michael__small_rule:n #1
  {
    \vrule
      height \dim_eval:n { \fontdimen 22 \use:c { #1font } 2 %+ 0.2pt
      }
  }
% Generate the little vertical rule from middle to top of the fraction rule
% in the appropriate style.
\NewDocumentCommand \michaelsmallrule { }
  {
    \michael__mathchoice:oooo
      { \michael__small_rule:n { text } }
      { \michael__small_rule:n { text } }
      { \michael__small_rule:n { script } }
      { \michael__small_rule:n { scriptscript } }
  }

\ExplSyntaxOff

\newcommand*{\strange}[2]{% with ad-hoc correction (kern) for horizontal space
  { \frac{#1}{#2\,} \kern-1.2pt \michaelsmallrule }
}

\makeatother

\begin{document}

$\Gamma_{ \strange{a+1}{1+\strange{x}{y}^{\strange{\alpha}{\beta}} }}$

\end{document}