Yes ,the derailleur needs a bit of movement left in it and the chain needs to be long enough.
There are two limitations involved here: A) one limiting the size of the largest sprocket and the other B) specifying a limit for the 'capacity' of the RD to wrap the spare chain.
B) first: If the chain is too short then trying to shift into large/large will either not happen and lots of noise, or the lower teeth of the largest sprocket catches the chain but as the sprocket rolls over the chain is not long enough, even with the 'return' (lower) section of the chain dead straight, you then have a strong chance of ride-ending damage. And if the chain does just manage it, it will not want to shift back down the cassette because the (in-theory) non-tensioned part of the chain passing through the cage/jockey wheels is under increased tension and will resist the sideways guidance of the jockey wheel - note that the cage is fully extended and therefore weaker as well. When in large/large the rule of thumb I use is that the chain is too short if the cage is pointing earlier than 4:30 (<135 degrees). Better to accept the penalty of too long a chain which will mean the RD cage folds on itself (when in small chainring / quite small sprocket) and the chain interferes with itself. Rarely any damage accrues and the noise prompts the rider to shift to a large(r) chainring.
The A) limitation can be addressed by a wolftooth option (as above) or similar once the 'b' adjustment screw adjustment (including reversal) has been fully exploited.
What I want to know is what
@Racing roadkill (K13133) is pointing at as he climbs Box Hill? The edge of the road?