/usr/lib/x86_64-linux-gnu/sasl2/libplain.so is in libsasl2-modules 2.1.27~101-g0780600+dfsg-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/sasl2/libplain.so.2.0.25
/usr/lib/x86_64-linux-gnu/sasl2/libplain.so.2.0.25