It was removed as noted in the changelog:
https://github.com/emscripten-core/emscripten/blob/1.39.5/ChangeLog.md#v1395-12202019
> Removed `timestamp` field from mouse, wheel, devicemotion and
> deviceorientation events. The presence of a `timestamp` on these
> events was slightly arbitrary, and populating this field caused
> a small profileable overhead that all users might not care about.
> It is easy to get a timestamp of an event by calling
> `emscripten_get_now()` or `emscripten_performance_now()` inside
> the event handler function of any event.
Fixes#34648.
435f86cf87
