What is that yellow square to the right of the main entry door of this Lufthansa A350?
It's a yellow square that says Willkommen / Welcome. In one case I have seen that additional text was written into the square with a black (removable?) marker.

Source
This website explains it like this:

A yellow welcome panel painted beside each of the passenger boarding
doors is now visible on each aircraft. The mark is a flat yellow
rectangle, with the word 'welcome' in both German and English,
accompanied by the iconic logo in the corner. What I love so much
about this - is that that bright yellow warmth is presented to every
passenger, right as they board every aircraft. Upon boarding, each
passenger is welcomed, setting the stage for the onboard experience
and hospitality that will follow. The crew can write personalised
messages to give a more human welcome to this otherwise mechanical
setting.