Attempting to use the optional chaining operator (introduced 2020) in #7474 raised an error when running npm run build:
This seems to be because falafel (used in one of the preprocessing steps) depends on an older version of the acorn parser (7.4.1, released 5 years ago).
Adding an override to package.json to bump the acorn version allows the build to complete successfully.
"overrides": {
"falafel": {
"acorn": "^8.1.1"
}