Since Mojang said they wanted to "give Birch Forests a unique identity" during Minecraft Live 2021.
On May 11, 2022, they confirmed they have stopped working on Birch Forests during an "Ask Mojang" video.

We're still waiting.

Since Mojang said they were going to add Fireflies to Minecraft during Minecraft Live 2021.
On May 11, 2022, they confirmed that fireflies were no longer in their plans during an "Ask Mojang" video.

This decision is explained because a single firefly's Lucibufagin is enough to poison and kill pet amphibians or reptiles.
They have done similar changes in the past with parrots and cookies, to (understandably) stop kids from accidentally poisoning their pets (parrots were actually tamed with cookies in 1.12 snapshots!)

Considering the amount of ambiance fireflies would add to the game, and that they seemed like a mostly finalized feature during Minecraft Live 2021, this decision struck the community as odd.
There is no reason frogs have to eat fireflies, or why a frog with no other food sources couldn't have a small chance to eat one and be poisoned to illustrate this point, like they did with parrots.

We're still waiting.

Since Mojang announced bundles during Minecraft Live 2020.

On November 17, 2021, they announced that bundles had been "put in the backburner".

Bundles were not announced as a part of the Trails and Tales Update.

We're still waiting.

On October 3, 2020, Mojang announced the Archaeology System during Minecraft Live 2020. On April 14, 2021, they announced that the feature had been postponed for Caves and Cliffs Part 2 during an announcement video.
On November 17, 2021, they announced that Archaeology had been "put in the backburner". Archaeology was not added to the game in the Wild Update.

On February 10, 2023, Mojang announced that archaeology would be coming to Minecraft 1.20.
An archaeology system was added to the game in Java Snapshot 23w07a, bringing this system to the game after two years in the backburner.

However, the archaeology system added to the game lacks many of the features shown during its announcement, such as dirt blocks changing size as they are brushed, overworld interactions to decorate clay pots, pots with 8 slots for decorative sherds, baking pots on top of a fire, and a more detailed clay pot model.
One has to wonder why Mojang stripped the archaeology system from most of its advertised features for release, as one would expect this system to be more developed and polished than its prerelease footage after over two years.

It took them over two years to release a half-baked update

Since the community voted to update Mountains during MINECON Live 2019.

On September 17, 2019, Mojang tweeted that all biomes in the vote would eventually get updated.
On October 16, 2021, they followed through, and announced that there would be a Swamps overhaul in the Wild Update!

Badlands weren't a part of the Trails and Tales update.
Mojang has the resources to add all vote options. It's just marketing through manufactured engagement on social media.

We're still waiting.

Since Mojang first released a "combat test" snapshot.
Their intention was to polish Java's "controversial" combat after the 1.9 update.

On August 15, 2019, they confirmed combat tests would be added when they were "good enough"
On November 20, 2019, they displayed commitment to improving PvP and PvE combat through these snapshots.
On August 7, 2020, they said they weren't ready to move to phase 2 of the tests.
On August 21, 2020, they said they thought they were getting somewhere with the tests.

They haven't released any new combat test snapshots or added these much needed improvements to any updates since.

We're still waiting.

Since Mojang said they had "a great idea" for fletching tables in the "next major themed update".

We're still waiting.

Since the community voted to update Taigas during MINECON Earth 2018.

On September 17, 2018, Mojang said other biomes in the vote would get updated too on a Reddit thread.
On September 17, 2019, they tweeted that they "haven't forgotten" about these biomes.

On October 15, 2022, Mojang announced the addition of camels to the game during Minecraft Live 2022.
The addition of camels came without any meaningful changes to deserts, despite there being a strong thematic link between the two.

Neither Savannas nor Deserts were a part of the Trails and Tales Update.
Mojang has the resources to add all vote options. It's just marketing through manufactured engagement on social media.

We're still waiting.

Since Mojang last said they were still working on the Plugin API for Java.
They later said they will never make something better than Forge, and official modding support doesn't exist yet.

We're still waiting.
(Background art from the All of Fabric 5 modpack)

Update 1.3.1 accidentally broke Minecraft's "Damage Tilt" feature.
On September 2020, they deemed the issue a "Won't Fix".

In October 2020, Shivaxi made a video explaining this bug and released a mod to fix it.

At 12:05PM on December 22, 2022, Mojang resolved the issue on their bug tracker.
On January 18, 2023, Java Snapshot 23w03a, the first release without this bug since its inception, finally made this fix accesible to players.

It took them 9 years to fix an obvious bug