The yew closes the fourth aicme and with it the original alphabet, and what happens next is one of the better-documented episodes in ogham's history.
Twenty is the whole of it
The ogham alphabet as it appears on the stones has twenty letters in four aicmi of five, and the design is transparently systematic: one to five strokes, in four positions relative to the stemline. That structure is what an alphabet invented at one stroke by one person looks like, and most scholars think that is what happened.
The twenty letters were fitted to the sounds of Primitive Irish. They worked.
The forfeda
Then the language changed, and Irish acquired sounds, diphthongs, and eventually the letters needed to write Latin loanwords, that the twenty could not represent. The response was a fifth aicme, the forfeda or supplementary letters, generally counted as five.
They are late, they are rare on stone, they are shaped quite differently from the original four aicmi, and the manuscripts disagree about their values. They are patches.
Why this is the useful fact
Because it shows the alphabet being used as an alphabet. Somebody needed to write a sound and there was no letter for it, so letters were added, imperfectly and inconsistently, in the way that happens to every writing system that is actually in service.
That is not how an oracle behaves. An oracle with twenty positions does not gain five more because the vernacular acquired a diphthong.
The yew
Iodhadh itself is one of the letter names with a good arboreal claim, and the yew is a genuinely significant tree in Irish tradition, the longest-lived, the churchyard tree, the wood of the household. The kennings gloss it with the oldest of trees and the fairest of the ancients.
As the last letter of the original row it is read in modern practice for endings that are also continuities, and for death understood as transmission rather than termination. That reading is modern. The tree is not.