Where have all the Y-wings gone?

Question

There is a significant battle in both The Force Awakens and The Last Jedi that require heavy bombardment on an enemy target.

In The Force Awakens it is the attack on Star Killer base, and in The Last Jedi it is the attack on the Dreadnought-class Star Destroyer.

Both attacks are eventually successful, but one thing I noticed was in both assaults the fighters sent did not seem to have the right type of vehicle to effectively take on their target. The attack on Star Killer base used primarily X-wing fighters whose firepower did not seem heavy enough to penetrate the oscillator, while the MG-100s seemed to be too slow/not maneuverable enough for the attack on the dreadnought.

So my first thought was, why not Y-wings? They had been used to great extent (mostly¹) sucessfully during the battles of Scarif, Yavin IV, and Endor. They are described as an "effective marriage between speed and firepower" which would have made more powerful than the X-wing and faster than the MG-100. It seems like the perfect fight!

So what happened? Was the manufacturer shut down, were other ships upgraded to render the Y-wing ineffective, was it an out-of-universe reason....

Where have all the Y-wings gone?²

_{1 - Yes, I know the Y-wing did not make the killing blows on either Death Star, but they still were the first choice.}

_{2 - To read to this tune}

Answer 1

Like the Rebellion, the Resistance was working with whatever ships it could find. Even the T-70 X-wings in its fleet were only in use because they were more common than T-85s:

Anticipating a conflict with the mysterious First Order, New Republic representatives formed the Resistance, and began obtaining starfighters in preparation for war. Because used and decommissioned T-70 X-wing (sic) were both available, affordable, and remained highly reliable, Resistance agents discreetly obtained as many T-70s as they could. Eventually, the T-70 X-wing became the signature combat craft of the Resistance starfighter corps.

[...]

When the First Order destroyed the Hosnian system, they not only atomized countless lifeforms on five worlds but also most of the New Republic Defense Fleet, including most T-85 X-wings in the Republic Navy. Desperate to prevent the First Order from committing further atrocities, the Resistance scrambled to acquire any available starfighters. Although they managed to obtain more T-70 X-wings, they may never again see the T-85.

_{Rebel Starfighters: Owners' Workshop Manual, p82 and p96}

While the Rebellion-era Y-wings were 50 or more years old by the time of the sequel trilogy, there were also updated BTA-NR2 Y-wings in production, which were eventually used at the battle of Exegol:

By the time the Alliance defeated the Empire, most operational Y-wings were well over twenty years old, and were skeletons of the original Clone Wars-era armored models.

After the Alliance transitioned to become the New Republic, Koensayr ultimately embraced its role in galactic revolution by restarting the Y-wing product line with the BTA-NR2 Y-wing.

_{Rebel Starfighters: Owners' Workshop Manual, p110}

Since this source describes the Resistance "scrambl[ing] to acquire any available starfighters" after the destruction of Hosnian, the implication is that the Resistance was only able to acquire Y-wings as they became available on the market, and only started doing so in earnest after the destruction of Hosnian in 34 ABY. This seems to be an in-universe explanation for why updated Y-wings appear in The Rise of Skywalker but not either of the earlier films.

Answer 2

The Y-Wing was an old and essentially obsolete design. The first Rebels episode of the third season, "Steps Into Shadow" set in 2 BBY, involved stealing Y-Wings that were in the process of being scrapped, and Hera considered them old then. The new trilogy is set 36 years after that.

Answer 3

The original trilogy xwings (T65 X wings) are old and outdated. The new trilogy uses T70 X wings which are newer, more effective fighters. If there is no newer model of Y wing, ordinance equipted T70s are likely more useful in those applications than outdated Y wings. The BS 17 resistance bombers also clearly held enough bombs to take out the dreadnought. I do not believe any other rebel bomber had that capacity.