mackel [1] filed a deletion request for mpv-build-git [2]:

The meson-build-option file of mpv is upgraded.the option "libplacebo-
next" is changed to "libplacebo".So it is told with "eson.build:1:0:
ERROR: Unknown options: "libplacebo-next".

[1] https://aur.archlinux.org/account/mackel/
[2] https://aur.archlinux.org/pkgbase/mpv-build-git/