She is Bangladeshi by birth until the age of 21, regardless of whether she ever applied for a passport. If they deem her a threat, they can either let her languish in a refugee camp or they can arrest her and put her on trial
I'm not sure it's that simple. From BD news 24:
"Shah Ali Farhad, who is a barrister by training, (and assistant to the Bandgladesh PM) gave a detailed explanation of the issue citing different references of the British as well as Bangladesh and international laws in his Facebook post before saying that Shamima is not a Bangladesh citizen either...."
He also said she is not a citizen of Bangladesh and that “dual citizenship, or citizenship by descent, is not an automatic right.
“It needs to be granted by the government of Bangladesh based on an application from the person, or her parents in the case of minors, seeking Bangladeshi citizenship by descent [Article 2-4 of the Bangladesh Citizenship (Temporary Provisions) Order 1972 and Rules 3, 4, 6, and 7 of the Bangladesh Citizenship (Temporary Provisions) Rules 1978].
“The provisions of the 1972 Order and the 1978 Rules have to be read together in harmony with the Citizenship Act, 1951 and the Citizenship Rules, 1952 to get a complete picture of the laws of citizenship in Bangladesh.
“One of the factors that the Government of Bangladesh looks at while granting dual citizenship is the applicant’s ties to Bangladesh,” Farhad said.
“Shamima Begum, never having travelled to Bangladesh and never having applied for a Bangladeshi citizenship, cannot be considered to “have a right” to Bangladeshi citizenship.”
m.bdnews24.com/en/detail/bangladesh/1595221