Source: Deleted on Reddit

We now only have 1/65536 of the unique world seeds compared to pre 1.18. The rest of these are exact copies of each other. There are only 2^48 unique seeds instead of 2^64. Another way of putting it is that only 0.00152587890625% percent of seeds remain. 

From finding interesting spawn points with the coolest terrain generation to share with your friends, to complex searches using the top 16 bits of the long type getting the fastest possible seeds; the community relies on the long type (support up to 2^64), the relationship between the 48 lower and 16 upper bits that makes a seed. Unfortunately, in the 21w37a, this has been reduced to only using Java random, which only uses 48 bits from the seed. This loses the ability to change the spawn point or biomes while leaving the structures, terrain, loot, Nether, and End the same. The seed finding community heavily relies on the full use of 2^64 seeds and the additional relation between the lower 48 bits (the "structure seed") and the upper 16 bits that determine biomes. In the current latest experimental snapshots, every seed you load will have 65535 other exact copies, so over 99.99% of seeds are redundant.
To put into context how few 2^48 seeds is, there would only be around 800 12 eye portals in the first ring, so the insane seeds you see in speed running WRs would not be possible with just 2^48 seeds.
 
Seed example of if you want an in-game example:

1

281474976710657

562949953421313

844424930131969

1125899906842625

1407374883553281

1688849860263937

1970324836974593

2251799813685249

2533274790395905

These are normally all different, but currently in 21w37a (+ many more are the same)

Side note: Shadow Seeds don't exist anymore either.