Warped and crimson fungus don’t grow into 3-block thick variants anymore

What is the 3-block-thick variant? Could you include a screenshot?

From what I know these types of "thick" huge fungi can only generate during initial world generation, but not with bonemeal.

You claim that they did in some snapshot. Which one was that?

The first snapshot

I looked in the changelog of all following snapshots and in none of them does it say that fungi no longer grow into a thick variant. So this is a bug

I found some in a world that was generated in 21w05b, if that helps anyone.

"Thick" huge fungi can grow from bonemeal in Bedrock, so this could be considered a parity issue.
I have personally never seen a "thick" huge fungi grow from bonemealing in Java in the release versions, having gone through hundreds of them.

can confirm that it was possible in early snapshots (20w08a) and doesn't work in the latest version (1.19 rc2 and also release 1.18.2)

Also it's important to note that:

Regarding the pinned comment by @Dylan C. quoting, "I looked in the changelog of all following snapshots and in none of them does it say that fungi no longer grow into a thick variant. So this is a bug", if you go to the first version which they don't grow to 3x3, the changelog has this bug fixed: https://bugs.mojang.com/browse/MC-171048 - this bug clearly states that the 'planted' field of huge fungi wasn't being utilised for differentiating between natural fungi (can grow to 3x3) and planted fungi (can only be 1x1). If you look in the code between these 2 versions, you'll see 20w09a added this line which confirms this change was definitely intentional:

boolean bl = !hugeFungusFeatureConfig.planted && random.nextFloat() < 0.06F;
this.generateStem(structureWorldAccess, random, hugeFungusFeatureConfig, blockPos2, i, bl);
this.generateHat(structureWorldAccess, random, hugeFungusFeatureConfig, blockPos2, i, bl); 

// And what's the last argument in 'generateStem'?
private void generateStem(WorldAccess world, Random random, HugeFungusFeatureConfig config, BlockPos pos, int stemHeight, boolean thickStem) {        
     Mutable mutable = new Mutable();
     BlockState blockState = config.stemState;
     int i = thickStem ? 1 : 0;
// And the same 'thickStem' variable is used during hat generation to make it slightly wider:
private void generateHat(WorldAccess world, Random random, HugeFungusFeatureConfig config, BlockPos pos, int hatHeight, boolean thickStem) {
...

This was most definitely patched intentionally and there's 0 evidence to believe otherwise.

is the owner if this report still apparent? I've done a bit of research on huge 3x3 huge fungus generation which i'd like to add 

Yes me the owner is still apparent and I am very happy my report as made it to a community consensus

I can confirm in 24w05a

I tested with 25 crimson fungus and 16 warped fungus and they all only made 1-block thick huge fungi

Edit: moved to already pinned comment for clarity

Regarding the pinned comment by @Dylan C. quoting, "I looked in the changelog of all following snapshots and in none of them does it say that fungi no longer grow into a thick variant. So this is a bug", if you go to the first version which they don't grow to 3x3, the changelog has this bug fixed: https://bugs.mojang.com/browse/MC-171048 - this bug clearly states that the 'planted' field of huge fungi wasn't being utilised for differentiating between natural fungi (can grow to 3x3) and planted fungi (can only be 1x1). If you look in the code between these 2 versions, you'll see 20w09a added this line which confirms this change was definitely intentional:

boolean bl = !hugeFungusFeatureConfig.planted && random.nextFloat() < 0.06F;
this.generateStem(structureWorldAccess, random, hugeFungusFeatureConfig, blockPos2, i, bl);
this.generateHat(structureWorldAccess, random, hugeFungusFeatureConfig, blockPos2, i, bl); 

// And what's the last argument in 'generateStem'?
private void generateStem(WorldAccess world, Random random, HugeFungusFeatureConfig config, BlockPos pos, int stemHeight, boolean thickStem) {        
     Mutable mutable = new Mutable();
     BlockState blockState = config.stemState;
     int i = thickStem ? 1 : 0;
// And the same 'thickStem' variable is used during hat generation to make it slightly wider:
private void generateHat(WorldAccess world, Random random, HugeFungusFeatureConfig config, BlockPos pos, int hatHeight, boolean thickStem) {
...

This was most definitely patched intentionally and there's 0 evidence to believe otherwise.