[Zuman]

The recent thread on this topic seems to be locked, but I was also interested as the Schiit Loki Max EQ I recently purchased came with RCA shorting plugs, and Schiit is a pretty no-nonsense company.
I found this by Paul McGowan of PS Audio: https://www.psaudio.com/blogs/pauls-posts/shorting-plugs#:~:text=The%20idea%20behind%20them%20was,might%20be%20quieter%20if%20shorted.
...and this on Audiogon, where several people had quite a bit to say about seven years ago: https://forum.audiogon.com/discussions/rca-shorting-plugs

[Letitroll98]

Back when we were discussing the Pioneer VSX D912 receiver, the one Paul Geddes measured as having the lowest switching distortion of any amp he tested and subsequently used as his demo amp when he first displayed his CD speakers, someone here told me you need to use shorting plugs on all inputs.  Which I did and noticed a definite decrease in electronic haze, a blacker black as is sometimes mentioned.  So I've been using them on all my components.  Super cheap if you make them yourself or buy the cheap ones on eBay.  Do not bother with the plain caps originally from Cardas with no grounding lug. They're useless.  Also a PSA to only install on inputs, never ever use them on outputs.  Unless you're trying to blow up your amp.

[SJ David]

As Paul McGowan stated, shorting plugs are a thing and have their purpose.  Paul says they used them in testing amplifiers. Schiit wants you to short the RCA input on the Loki but only if you use the XLR connector with a single-ended signal. I used them many years ago on equipment with dual phono or tape inputs. Can't say what result you can expect on shorting unused line-level inputs in a more current, well-designed/engineered preamp to begin with.  I agree with Paul's sentiment when he says he has never spent the time to determine if there is an audible difference.