[_flux]

As it says:

I-JSON (short for "Internet JSON") is a restricted profile of JSON designed to maximize interoperability and increase confidence that software can process it successfully with predictable results.

So it's not JSON, but a restricted version of it.

I wonder if use of these restrictions is popular. I had never heard of I-JSON.

[rcxdude]

I think it's rare for them to be explicilty stated, but common for them to be present in practice. I-JSON is just an explicit list of these common implicit limits. For any given tool/service that describes itself as accepting JSON I would expect I-JSON documents to be more likely to work as expected than non-I-JSON.