Internally, Ignition is written in Java, so uses the Java standards to handle integers. That is Big Endian, but that doesn’t matter for any end user.
What does matter is what the individual drivers support to communicate with other devices. If you use the Siemens driver, it’s made for Siemens, so as long as all data types are available, they will be translated correctly (we have used the Siemens driver before, and it doesn’t give issues with data types).
It’s all defined in the addressing: https://docs.inductiveautomation.com/display/DOC79/Siemens#Siemens-ConfiguringSiemensAddressing
I don’t get why someone is interested in endianness unless you have to work on the actual driver code.
UPDATE:
Apparently, Siemens also used Big Endianness for most data (https://support.industry.siemens.com/tf/WW/en/posts/s7-1200-big-or-little-endian/143030?page=0&pageSize=10), with the one exception of optimized block data.
The Ignition documentation says you must turn off the optimized block data. So that probably means the driver only uses Big Endian encoding, and your client probably wants to know if they can enable the optimized block data or not.