Gstreamer shout2send plugin can support converting to webm format, but only if libshout-2.3.0+ is available. See bug #1562233
lastest libshout is currently 2.4.1
libshout 2.4.3 released few months ago, supports OpenSSL 1.1 and improved WebM/Matroska support.
ezstream-1.0.0 requires libshout with WebM support. That means libshout ≥ 2.4.3.
I upgraded this package.