I definitely support efforts to use more standardized language in these specifications, thanks.
One meta-comment though: For messages like this announcing proposals, as well as the pull requests they reference, it would be very helpful to always provide a clear and concise description. Otherwise it may take each reader several minutes to determine whether this is something they can contribute to, and uncertainty may also decrease participation.
In this case, for example: “Update GTFS specification using standardized definitions of terms like MUST, SHALL, or SHOULD, following RFC 2119."
Just to clarify the sources of uncertainty on my first reading: the expression “update GTFS Schedule” seems to be referencing the data in a feed rather than the specification text, and focus on the arrival/departure times rather than everything else in GTFS. More importantly, most people won’t remember what exactly RFC 2119 is about (and really, people in the wider transportation field might not even know what an RFC is). Even after opening the RFC text it takes some reading to be certain.