As Flight Detent said modern types (well certainly the Boeings, even the heavies) can be hand flown OK at high level, nothing really has changed, the problem is a legal one that is down to what Johnny FP has said, i.e. Reduced Vertical Separation Minima (RVSM)...
With vertical separation at high level reduced to 1000' (it used to be 2000') there's less of a buffer to cope with the inevitable plus/minuses around your assigned altitude that you will get on occasions when hand flying at high level. As a result various authorities/companies ops manuals pretty much forbid handflying except in exceptional circumstances/emergencies at high level.