Interesting, I had to check their ToS and found this which seems a bit vague:

"[...]use the Services to store or transmit any inappropriate content, such as content that: (i) contains unlawful, defamatory, threatening, abusive, libelous or otherwise objectionable material of any kind or nature" [1]

(My emphasis)

[1] https://keybase.io/docs/acceptable-use-policy