Looking at the Thatcham Method manual, it does in fact, form the bottom edge of the windscreen aperture.
Although the panel is listed, it doesn't include a time or method for replacement. Which suggests that if it is damaged by an accident that the car is fundamentally unrepairable as to distort it would mean damage that can only safely be repaired with a new shell. Which incidentally would require a new bulkhead assembly (the rusty part) as it isn't a part of the shell.
This manual is only for accident repair methods and costings, and primary external shell panels/crumple zones rather than the inner structure of the shell. Therefore it doesn't include corrosion repair which obviously falls into restoration territory...
Removing and refitting everything to access the panel is 41 hours assuming that the engine stays in. Plus another 24ish hours to remove it from a (complete) donor car.