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It applies it to inscription envelopes specifically, by testing for the existence of the bytes for "OP_FALSE OP_IF" in the witness

And the limit is the same 83B?

I think so. Here is the relevant code snippet: https://github.com/bitcoinknots/bitcoin/blob/eeb9cc1120661d0e9fd28ddb6fef2c04992a4666/src/script/script.cpp#L329 It is a function called DatacarrierBytes and I don't know c++ all that well but it appears to match transactions containing OP_FALSE OP_IF or OP_RETURN, and counts the number of bytes they carry as a payload. I assume this function gets called wherever the data arrives filter is applied, which I suppose is probably in policy.cpp -- though I haven't checked

I don't know enough either, but I wonder if the is used also on inputs, or just outputs.