It's been pretty easy because most updates are mainly just new mobs and they spawn on existing terrain. I pruned/trimmed the world only once, maybe about a decade ago, because we wanted better terrain and caves than what was originally generated. It left a big cube of the original map and everything around is relatively new. I haven't had to do it since. We just go further and further for new chunks now.

For example, I just updated to 1.21.11 and now there's just nautili in our waters. This world started well before bees, horses, polar bears, turtles and all that, but after updates, they are just there. It makes a somewhat weird mix of old terrain with new mobs, that gets progressively newer the further we go from the spawn point. Like, villages that are close were generated before professions and trading, and are very simple and boring. But as we venture deeper into the world, they start to look like modern villages, with profession blocks and trading now possible.