Is there an Ace Combat 8 demo or public beta?
No public demo or beta has been announced in the currently cited official sources.
Confirmed explanation
Preview sessions for media and paid Deluxe early access are not public demos. A future test could still be announced, so this answer is a dated status rather than proof that one will never exist.
What is not confirmed yet
Download timing, progress carryover and supported platforms do not exist until a demo or beta is announced.