OpenBSD 5.7 is no longer officially supported.
As described in the official FAQ's section on
flavors,
any version of OpenBSD is officially supported for only one year. OpenBSD 5.7 ceased to be officially supported as of spring 2016 with the release of OpenBSD 5.9. The latest version of OpenBSD to be released as of this date is OpenBSD 6.1.
You may receive a response to your question, but be beware that the software you are running in considered ancient, & outside of what project developers have any interest in supporting. Although this site is not officially affiliated with the OpenBSD project proper, we try to stay in line with their policies.
We highly recommend you either upgrade, or perform a fresh installation of a newer version.
From personal experience, I know that
www/youtube-dl frequently needs to be updated as the access methods allowed by YouTube are frequently modified.