Story #63815: Reduce communication between server and client
Remove Last-Modified response header from cached output
The current state of last-modified is broken by design.
Furthermore it is the question if we should support this properly at all in the future, since well, the whole page has to be rendered anyway to find out if it 'was-modified-since' so there is no benefit on server side (and not on client side either). Supporting that would also mean that we need to track/hash basically everything, so it would be a really evil cross-cutting-concern that affects extensions, content, files, everything.
A document is added to this issue explaining the whole situation