Plasma Wayland Protocols 1.14 introduces kde_external_brightness_v1 as a new protocol for externally-controlled display brightness. For HDR content, content-adaptive brightness, and other purposes the Wayland compositor needs to control the brightness of backlight devices. With this new kde_external_brightness_v1 protocol is for achieving just that so the brightness setting can then be handled by a process outside of the compositor like KDE's PowerDevil.
With the updated kde_output_device_v2 protocol, there is also now support for setting the brightness in SDR display mode.
With this new release, org_kde_plasma_window_management adds support for setting the client geometry of windows without decorations. The intention here is on better coordinate-based click support within the Selenium-AT-SPI code.
More details on the Plasma Wayland Protocols 1.14 changes via KDE.org . Plasma Wayland Protocols 1.14 is required for the upcoming Plasma 6.2 beta release.