It will depend how exactly JSON parsing is configured. You can set INDEXED_EXTRACTIONS=json in props.conf and then the JSON fields will be extracted on the indexer, or even earlier on a heavy forwarder if you are using one.
So if you are using INDEXED_EXTRACTIONS=json then I think you need to change the limit on all indexers and heavy forwarders. In this case data that is already in your indexes will not benefit.
On the other hand, you can also do JSON parsing at search time using KV_MODE=json in props.conf, the spath command or the spath() eval function. If you're doing any of these then you'd want the setting on the search head. It's conceivable you could also need the increased cutoff on the indexers as well because I don't see why JSON parsing couldn't be delegated to the indexers in a distributed environment.
Obviously for JSON parsing at search time any changed cutoff will apply to previously indexed data as well as data indexed after the change.
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