/r/videography - "A community for videographers of all skill levels in fields such as weddings, events, live performances"

/r/VideoEditing - "A subreddit for amateur, hobbyist, and prosumer editors to meet."

/r/editors - Sort of the professional equivalent of the above. For professional video editors. "A subreddit centered around postproduction."

/r/Filmmakers - "Filmmakers, directors, cinematographers"

/r/Editing - "Post production articles"

Software-specific

/r/AfterEffects - for discussing AfterEffects

/r/kdenlive - For discussing Kdenlive

/r/blender - For discussing Blender

r/davinciresolve - For discussing DaVinci Resolve

YouTube-specific

/r/NewTubers - For small, emerging, and new YouTube creators

/r/SmallYTChannel - A sub for smaller YouTube content creators to share and request feedback and tips

/r/PartneredYoutube - A sub for creators in the YouTube partner program

Hardware

/r/buildapcvideoediting - A very niche subreddit for advice upon building a rig / PC specifically for the purpose of postproduction and video editing

Platform-specific

/r/Linux_Filmmaking - For filmmakers that use GNU/Linux

Tangential

/r/photography - about photography

Hiring

/r/VideoEditingRequests - Post video editing requests for free / payment