I opened a rebar3_ex_doc issue to track this (Update to allow generating documentation from OTP 27 on · Issue #509 · kerl/kerl · GitHub), which is replicated by a PR (Test for OTP matrix by paulo-ferraz-oliveira · Pull Request #80 · starbelly/rebar3_ex_doc · GitHub). Feel free to discuss there, too, since this seems more plugin-specific than OTP-specific.
Edit: it seems, at this moment, we only need a rebar3_ex_doc release and the issue should be gone 
. I updated CI so as to not use Hex’s rebar3_ex_doc (but the very version we build), and it shows no errors. I didn’t dig into why this is, but it seems something changed in ex_doc from 0.30.7 to 0.31.1 that made it so 