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Are they optional¹ and are there alternatives to them? If the answer is yes, then there is a problem. 1 - I imagine "optional" can be the wrong word, maybe the correct is "universally adopted".
Maybe a more concrete example will answer your question. The Wayland protocols are found here [1]. Your compositor will implement some of these protocols and not others. If I’m writing an application I will; possibly through some toolkit, ask the compositor what features it supports and configure how my application works accordingly. This isn’t particularly different to say, an toolkit not supporting a particular for…
So... I was one of those people rooting for anybody to replace X11 when wayland launched. But given the current situation, I'm just hopping that Red Hat finishes dying off before I have to work with somebody stupid enough to set some GUI system that I need to access, but only runs on a computer where I can not run a GUI.