Hi there!
We also use today 2024 
ANSI-C as preferenced language within Automation-Studio.
Means:
- All our base-libraries we use for example for our AS-projects are programmed in ANSI-C.
- If our customer whant’s to implement the machine-level integration with IEC-ST (example only) there is no limitation using the ANSI-C libraries we provide.
= languages can be mixed
= some “minor” restrictions / limitations are present and documented within AS-help
So the simple answer to your questions is: don’t hesitate to use ANSI-C with AS as it is supported by B&R.
Good luck! Sämi, Team RSB