As mentioned in the comments, plain text keys aren’t bad because they are necessary. You have to have at least one plain text key in order to be able to use encryption
The link is broken, but this is apparently an issue with Signal Desktop, not regular Signal. The proposed solution does not work on Windows: https://www.electronjs.org/docs/latest/api/safe-storage
[…] content is protected from other users on the same machine, but not from other apps running in the same userspace.
It’s unfortunately about the best you can do on Windows.