If it's latest chapter, please do add a spoiler tag in front of the title ([spoiler] or something), so people like me who still waiting for the scan or official translation can avoid.

Not a mod, but if I was to post, I see no rule against it, and from the fediverse softwares I'm familiar with, both Lemmy and Mbin have spoiler hiding features, and Mastodon propagation only shows the attached image when available, the title and the link to the original post, so I wouldn't think there's any major risks.

Sidebar for the community says "be wary of spoilers" but there's no mention of a rule against them. For courtesy, I'd probably keep details minimal in the title but include a warning (e.g. 'S04E20 "Episode Title" discussion [spoilers]'). There's also spoiler tag markdown that you can use to hide the details unless a user chooses to view them:

The Spoiler
I don't know, some pirate stuff.