How to typeset asymmetrical confidence level limits / uncertainties?

Here's a way to stack asymmetrical limits using macro Vectorstack from package stackengine (based on further clarification you provide in comments to your question):

Output:

Code:

documentclass{article}
usepackage{stackengine}
begin{document}
[
  %setstackgap{L}{baselineskip} % Uncomment and change this if you want to adjust vertical spacing between stacked content
  A = B 
  enskip Vectorstack{+0.60 -0.59}
  enskip textrm{cm}
]
end{document}

I propose this solution using nccmath with the mrel.

documentclass[12pt]{article} 
usepackage{nccmath}
begin{document} $L=+0.39!mrel{+0.60[.12ex]-0.59} mathrm{cm}$
end{document}

---

Increasing the value of [...ex]:

documentclass[12pt]{article} 
usepackage{nccmath}
begin{document} $L=+0.39!mrel{+0.60[2ex]-0.59} mathrm{cm}$
end{document}