Why was Ossiriand spared from the sinking of Beleriand?

Question

From Hithlum to Angband to Maglor's Gap to the Mouth of Sirion, Beleriand was sunk with the exception of Ossiriand (Lindon, as it is known as in the 2nd and 3rd Ages). Was it because there was no damage to it and hence the Valar saw no need to sink it like all the rest of the lands west of it?

Answer 1

As ATB remarks in a comment on the question, the Valar didn't want to sink Beleriand. It just so happened that Morgoth was so powerful, the destruction of Beleriand was an unavoidable consequence of defeating him¹; we're told in The Silmarillion that:

[S]o great was the fury of those adversaries that the northern regions of the western world were rent asunder, and the sea roared in through many chasms, and there was confusion and great noise; and rivers perished or found new paths, and the valleys were upheaved and the hills trod down; and Sirion was no more.

^{The Silmarillion III Quenta Silmarillion Chapter 24: "Of the Voyage of Eärendil and the War of Wrath"}

So the reason Ossiriand is spared is quite simple: Morgoth had no significant forces in Ossiriand; in fact, we're told this as well, following the siege of Angband (emphasis mine):

[The Orcs] passed over Gelion with fire and terror and came far into East Beleriand. Maglor joined Maedhros upon Himring; but Caranthir fled and joined the remnant of his people to the scattered folk of the hunters, Amrod and Amras, and they retreated and passed Ramdal in the south. Upon Amon Ereb they maintained a watch and some strength of war, and they had aid of the Green-elves; and the Orcs came not into Ossiriand, nor to Taur-im-Duinath and the wilds of the south.

^{The Silmarillion III Quenta Silmarillion Chapter 18: "Of the Ruin of Beleriand and the Fall of Fingolfin"}

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¹ I discuss the reasons for this a little more completely in my answer to Why did the Valar abandon the Moriquendi and Men?