Removing a mandatory or optional property in the input data : breaking change ?

[Kourtakis]

Removing a mandatory or optional property in the input data (request body) is NOT recognized as a breaking change in the tool.
Why not ?

[danilofuchs]

I believe this is not a breaking change. If you remove the need for a field, if old clients send that field, the server should just ignore it and work as usual.