you can keep playing your world for good aslong as you never explore too much of the whole map. its very easy to avoid ashlands and deep north areas by simply never going all the way directly north or south to the edge of the map.

new enemy spawns happen regardless of how old a world is as enemys are spawned in dynamically.

so just explore into east or west in a huge cone and youll get everything while also leaveing tons of unexplored map for later.

they wont make THAT many additions to existing biomes anymore at this point probaly. the most important part is to not visit ashlands or deep north too much. as already explored areas of a map will not update anymore even after the update since they already got generated.

only way around that is to cheat with mods. but that can have some risk to it depending on world layout and bases/spawns.