Stop Chunk Loading

How do you stop chunk from loading when a player reach a certain limit? for example, maximum x coordinate is 128 and z coordinate is 128 also.

You can't. But you can extend the McRegion/Anvil level provider and return dummy chunks rather than loading/generating them

i was planning to create a world that its like the old one thats not unlimited.

Then rewrite PocketMine or make another server software. PocketMine forces infinite worlds
I mean, you cannot make it like old maps by not loading those chunks. You still have to send dummy chunks.
Therefore, the best method to do so is to send your own packets to the client and block the DataPacketSendEvent for FullChunkDataPacket.

im believing its possible but its just that we have no idea what to do with it. hoping @shoghicp can confirm this.

As I remember, the server sends data that it is an infinite world not an old world before we can handle it.

Chunk Request packets are never sent by the client :/