Sure, here it is: https://github.com/bitcoinknots/bitcoin/blob/28.x-knots/src/validation.cpp#L4458-L4469

I should add that this is optional, but default on.

sedited

Making people upgrade faster by hard-forking older clients off the chain, like knots does, is a dangerous path to tread on for the entire project.

Interesting. Do you have a code reference for this hard forking - I can't seem to find it in the knots code. Or do you mean with "hard-forking" that if 

 would at once implement policy tightening, there would be transactions stuck forever?

bitcoin

A Comprehensive OP_RETURN Limits Q&A Resource to Combat Misinformation

tidwell

> Making people upgrade faster by hard-forking older clients off the chain, like knots does, is a dangerous path to tread on for the entire project.

Interesting. Do you have a code reference for this hard forking - I can't seem to find it in the knots code. Or do you mean with "hard-forking" that if **everyone** would at once implement policy tightening, there would be transactions stuck forever?