In newer versions of `biblatex`, it has changed dependency to `dev-texlive/texlive-plaingeneric` as it has replaced `dev-texlive/genericextra` in 2017 version of TeXLive. As biblatex-3.4 is still the newest stable version of `dev-tex/biblatex`, it should be migrated to be compatible with the newest stable TeXLive version.
sorry for ignoring this, this has been fixed in biblatex-3.4-r2 by aballier as he updated the texlive dependencies.