I am running OpenBSD 6.8 and would like to upgrade httpd to the -current version to enjoy a new feature.
Is this possible without upgrading the entire operating system?
No it's not possible. In OpenBSD you cannot mix release/stable with other versions because other versions depend on other parts of the OS, eg. API/ABI. Sometimes developers add/remove/change some internals so an app depending on such internals won't work.
OpenBSD doesn't keep backward compatibility like Linux.
-release or -stable to -current, it's tricky to bring it back to e.g. -stable (which is what you want to run your production systems on). Better to just wait for the next -stable to come around.
httpdbut not upgrading the rest of the system.