This is a temporary hack, until this gets merged:
https://bugs.archlinux.org/task/76710?project=5&string=obs-studio obsproject/obs-studio#7475
The github action in this repository creates a tarball with alle the plugins (including websockets). To install it, run:
git clone https://github.com/tbocek/obs-source-all.git
MAKEFLAGS="-j$(nproc)" makepkg -Acs
sudo pacman -U obs-studio-31.0.0-2-x86_64.pkg.tar.zst
This version includes Websockets, but disables NVenc.