Last commit from 3 years ago. Qt6 porting PR was filed, then closed a year later. Is there any hope for this package?
Actually, this has been superseded by https://github.com/input-leap/input-leap. There was lots of activity for ages, but no actual releases. In the meantime, I switched to Wayland, which barrier doesn't support, and have made do with net-misc/netevent ever since. I gave up checking on input-leap, but I now see they have actually done some releases back in October, so I'll arrange a bump and package move.
input-leap is in ::guru, I could update the old ebuild with my local version and you could see if that suites you.