Toggles HDR automatically. Depends on https://github.com/dyphire/mpv-display-plugin.

hdr-toggle.lua

local function toggle_hdr_display(target)
    local hdr_status = mp.get_property_native("user-data/display-info/hdr-status")

    if hdr_status and hdr_status ~= target then
        mp.commandv("script-message", "toggle-hdr-display", target)
    end
end

local function auto_switch(name, value)
    if not value then return end

    local target = (value == "pq" or value == "hlg") and "on" or "off"

    toggle_hdr_display(target)
end

mp.observe_property("video-params/gamma", "native", auto_switch)

mp.register_event("end-file", function(event)
    if event.reason == "quit" then
        toggle_hdr_display("off")
    end
end)