I noticed that in 1.13, it's all becoming flatten id. It seems that the world save file format has changed a lot.

I made a test on the dev version of spigot 1.13. After upgrading, it requires user to execute with '-force-upgrade' parameter to transform old save file for the first time. It might spends a long time, but after that you'll get all of your world worked on 1.13. Will forge provide something similar? Or should that be the job of mod developers to provide a way to migrate save from 1.12 to 1.13? Or should we just give up our current save, like what has happened in the upgrading from 1.7.10?

 

There's also something on https://gist.github.com/williewillus/353c872bcf1a6ace9921189f6100d09a   which says:

 

1. Make a branch off of 1.12, and flatten all your items there first. Stop being scared of using more ID's. This is useful because you can still run the game and make sure everything you've done so far works. Make a DataFixer to migrate saves for you (or just decide to break world compat). (easy, tedious) 1a. By request from Mezz: Update JEI integration to remove all uses of deprecated methods in 1.12. This is so he can remove those methods directly in 1.13.

It really depends on how heavily the mods used metadata. Some may not need any works. Some may have stuffed all their items/blocks into metadata to save space