I think today the only reason to need to know linux is actually backend dev and embedd in the sense of small pcs with custom uis.
With the amount SaaS today most of the backend services can be bough, so that also lowers the amount of developers who need to know linux.
As a comparison I don't have the skills or need to develop firmware or other software that directly interacts with hardware, but having some high level knowledge of how CPU, RAM and I/O physically operate has been very useful in designing good programs. I feel comfortable arguing that investing a small amount of time in basic linux knowledge is going to be a net benefit for virtually any professional developer.