I recently created an Ubuntu 18.04 on Azure to run a test service I work on in my spare time and noticed that the /etc/rc.local file doesn't exist by default anymore. There are other (probably preferred) ways to start applications when the server boots by rc.local was in my opinion, the absolute easiest.
TL;DR => etc/rc.local still works, you just have to create it and then give it execute permissions. Remember that it is a shell script so it will need to be setup as one.
/etc/rc.local example
#!/bin/sh -e
cd /home/admin/test-service
./start-service &
sleep 10
# Always leave this line here
exit 0
Permission required for /etc/rc.local
sudo chmod 755 /etc/rc.local