Likely they can lookup your passport photos.
Since denying entry to a citizen can be a constitutional violation in many places, they’ll have some criteria for the documentation actually required.
Sending you back to the embassy will just move the problem elsewhere.
Things get dicier when there’s no records of you being a citizen (e.g. you were born elsewhere but acquired a citizenship because you were born to a citizen without registering it in any way).
Perhaps because you successfully applied for British citizenship in the UK forty years ago but the government deleted all the records! Several instances of this came to light in the Windrush scandal a couple of years ago.
The moral: if your citizenship is even slightly interesting you need to keep your passport current.
Getting denied boarding because I am a citizen.
By lineage, I’m a citizen of an EU country.
And the EU is launching a “not a visa but sounds a lot like a visa” travel authorization program for visa exempt passengers.
Q1 will probably be: are you a citizen of any of these EU countries?
Legally, I’ll be obligated to trace all this lineage back, through countries that no longer exist and get a lot of paperwork.
Presumably I can’t renounce said citizenship until I prove that I have it first...
The lineage is direct: both parents are citizen’s of that country’s successor.
But maybe it’s more discretionary in reality.
There’s always people willing to give you a hard time even if you’re in the right.
At least in Canada I know how to waste a lot of their time in return.
It doesn’t say “can acquire”, just “acquires”.
Not unlike an American or Canadian born abroad to natural-born Americans or Canadians.
How can you prove who you are without an ID? Biometrics? Is logging in to Facebook enough?