My company is WFH right now, but my team is distributed across 4 cities anyways. I've mentored a handful of junior developers and it seems to have worked well, however, you really need a culture which supports it. Active video/screen sharing time, solid onboarding documentation, RFCs which describe processes, etc.
Good technical mentorship relies on organizational will to dedicate the time and resources needed to make it happen. Documentation(wikis) and tutorials need to be up to date, junior engineers need properly scoped tasks with clear success criteria and access to senior engineers to provide guidance. I don't think being physically adjacent is necessary.
Do y'all feel that a trend towards remote work benefits mid/senior level employees at the cost of the development of junior employees ?
IMO, it is the same difference as in-person schooling vs online schools. As convenient as WFH is, it really isn't conducive to hands on mentorship.
Tech companies might not mind it, since the attrition rate in tech at the junior level is incredibly high anyway.