Strange .userchain file on my Mojave desktop
The most likely cause of the hidden .userchain text file is the installation of SampleTank software, up to and including version 3 (the current version as of this writing).

I'm not clear on why this is created by the software, but if you're continuing to use the software then I would leave it be as it's basically taking up no space and doesn't cause any known problems in macOS.

However, if you no longer use the software, it's safe to remove. If in the future you launch it again, it'll just recreate the .userchain text file.

Likewise, if you've totally removed the software, it's also safe to remove.