[SalesForce] Filter for datatable in LWC

Try to modify your code.

// for(i = 0 ; i< list.length(); i++)
for(i = 0 ; i< list.length; i++) {
  //alert('a' list[i].checked);
  alert('a' + list[i].checked);
}

In my environment, alert works without problems.

I've found a tags with an onclick event to be problematic too.

So, this is just a suggestion, but try a different approach. Create a span with an onclick event and style it like an a element. This has been successful for myself in lightning communities.