Just to clarify: I think an older version may have used React, but I just double-checked and current Reddit is definitely a Lit app built with web components. You can tell based on the comment placeholders used by the rendering engine as anchor nodes and by the prevalence of dashed-elements in the markup. Here's an example:

  
          
          Send a GIF
  

They seem to call the current architecture "shreddit" internally for some reason.