post here 100%: /r/Breadtube, /r/ChapoTrapHouse, /r/LeftWithoutEdge, /r/Socialism

Maybe post here but think about it first:

/r/EnoughPetersonSpam, /r/EnoughIDWSpam and /r/EnoughMuskSpam for videos responding to Jordan Peterson, the intellectual dark web in general or Elon Musk respectively.

/r/TheMajorityReport, /r/TheDavidPakmanShow and /r/SecularTalk for videos that reflect the topics discussed in the left-wing YouTube news sphere i.e. US domestic and Foreign policy (Majority Report leans much further left politically and socially than the others so more socialist or socially left leaning material would be a better fit there than the other two).

/r/Hbomberguy, /r/ShaunandJen and /r/PhilosophyTube. /r/ContraPoints don't like non-Contra related content as much so posting in their off-topic megathread is better option for there.

/r/Fuckthealtright and /r/beholdthemasterrace for content criticising the alt-right and white supremacists (the latter sub is more for straight mockery)

/r/PoliticalRevolution and /r/WayOfTheBern for videos relating to Bernie, AOC et al

/r/LGBT and /r/traaaaaaannnnnnnnnns for videos corresponding to those topics

Finally /r/mealtimevideos is a less assured but still worthwhile option. It's not a universally left-leaning sub, but when videos by channels like ContraPoints have been shared there they've been very popular, so to those prepared to face the possibility of more right-leaning commenters I do still recommend it. Still better for content that might reach across the political aisle a bit more rather than straight up partisan content.