According to Yuki's comment[1], we might remove [Custom] extended attribute for MessageEvent.data. [Custom] is meant for the case that the bindings code generator doesn't meet the requirements of Web APIs, so that you can write your own bindings.  Unless you really need, we SHOULD NOT use it.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1226774/4/third_party/blink/renderer/modules/payments/payment_method_change_event.h#45