I'm the lone super-mod of Monstrum the game. The most of the video in this category were approved back in 2017, when more players were active. I've checked some runs today and realised some source video (on Twitch, mostly) don't play at all (just black screens). Is this fact a reason to delete this runs from the result tables as far as these results don't have a source video anymore?
No, once the run is approved it should stay on the leaderboard (Unless someone report the run for being impossible to beat and requires a second verification and the runner can no longer provide a video). But for future reference it's always recommended to upload video on Youtube or something where it doesn't get deleted.
In the game where I'm most active, we delete runs that no longer have video proof. No point in having it there if the person can no longer prove that he/she got the time that they submitted.
Like @dripping , I think there's no reason to keep a time on the leaderboard if the video has been removed. It's the responsibility of the runners to never privatize their speedruns. It's also sketchy to privatize a run. Someone could upload a run, get it approved and privatize it before the rest of the community detects the anomaly. See what happened to the Minecraft community where fake runs were detected years after being approved. That wouldn't have been possible had those runs been privatized. Just my 2 cents.