This post discusses the fading custom of writing the Tetragrammaton in siddurim with the י–י digraph, rather than the increasingly common full spelling, which I will not put here, as I suspect that most users of this forum should already be familiar with it. While the author brings one source which he links, most of this article relies on conjecture and scans of older siddurim, including those of the Shelah and R' Yaakov b"r Tzvi of Emden.
What sources (aside from those in the article) discuss the use of the tetragrammaton or lack thereof in siddurim and other works besides Tana"ch?