Comment 9 for bug 872006

As workaround, you can use the SEARCH function for now.

Knowing the key of the track currently playing, just enter that key into the search, and get all harmonically compatible tracks. You can also combine that with other operators like BPM: , to get more granular results.

* Supports fuzzy matching of key searches using the ``~``operator. The following example lists tracks with harmonically compatible keys to C# minor.
~key:c#m

https://mixxx.org/manual/latest/en/chapters/library.html
Note: Looks like the link to them manual is currently down