This was added in T212124: Consider adding decoding=async to our img tags but never ported to Parsoid (see https://gerrit.wikimedia.org/r/481883) and it appears it is here to stay.
Change 720136 had a related patch set uploaded (by Arlolra; author: Arlolra):
[mediawiki/services/parsoid@master] Add \"decoding=async\" attribute to img tags
Change 720136 merged by jenkins-bot:
Change 720809 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/vendor@master] Bump wikimedia/parsoid to 0.14.0-a18
Change 720809 merged by jenkins-bot: