I recall that Concorde used to be able to cruise climb throughout its entire flight as the fuel burned off and it got lighter and therefore more advantageous for it to go higher, because it was the only thing operating up at its altitudes (other than maybe an occasional U2 or SR71). It didn’t have the ATC constraints that other transport aircraft do.
That said, I seem to recall seeing somewhere that aircraft crossing the isolated south pacific on non-stop flights from South America to Oceana are also given blocks of altitudes because there are very few of them down there to conflict with one another, so they can be a bit more flexible.