A short answer is — it's a Debian bug, report it to Debian, please.
A full explanation is in this comment.
A fast solution is to upload the old libmariadbclient.so.18.0 from Mariadb 10.1 installed from debian repo and make a symlink to libmariadbclient.so.18.
This is the currently the only way I'm able to use pure-ftpd with Mariadb 10.2
Alternatively, you can build pure-ftpd from sources (apt-get build-dep … apt-get source … dpkg-buildpackage). While I did not try that myself, I've seen that pure-ftpd detects the availability of my_make_scrambled_password automatically, so it should not use that function if the library does not provide it.
Unfortunately it seems not really to be solved: I installed MariaDB 10.4.4 and then pure-ftpd on debian and got same error message:
pure-ftpd-mysql[30599]: Starting ftp server: Running: /usr/sbin/pure-ftpd-mysql -l mysql:/etc/pure-ftpd/db/mysql.conf -l pam -O clf:/var/log/pure-ftpd/transfer.log -u 1000 -8 UTF-8 -J HIGH -E -B
pure-ftpd-mysql[30599]: /usr/sbin/pure-ftpd-mysql: relocation error: /usr/sbin/pure-ftpd-mysql: symbol my_make_scrambled_password, version libmariadbclient_18 not defined in file libmariadbclient.so.18 with link time reference
I cant remove or purge pure-ftpd because "Package 'pure-ftpd' is not installed, so not removed".
Retrying it error messages like starting pure-ftpd 
It is not solved, see above the resolution says "Won't fix". It's not MariaDB bug, it's not pure-ftpd bug. It's debian bug, debian builds pure-ftpd in a way that inroduces this error, even though pure-ftpd can be built so that this error won't happen.
The first comment under the bug has a link to the detailed explanation.