I guess that was Ubuntu 18 indeed, on Ubuntu 16 I've never seen anything other than 1.0.2. See https://packages.ubuntu.com/search?keywords=libssl&searchon=names&su...
I can confirm that after downgrading libmysqlclient20 to `5.7.26-0ubuntu0.16.04.1` the problem seems to be gone - at the very least my sipp stress test no longer manages to cause any deadlocks.