did you use a random or pre-selected seed?
did you allow friends or play solo?
will you reset chunks to make room for new biomes/terrain features with updates?
what mods did you use?
what difficulty did you select?
what gamerules did you change?
how long has it been going?
Closest thing to a forever world I did was in the middle of beta, back when biomes were more the product of smoothly varying numbers than hard-coded distinct regions with borders. Random seed, medium difficulty, solo, no mods, and gamerules weren’t a thing. But it’s long lost now.