No Tubey - you become eligible for Bangladeshi citizenship, and there should be no impediment to getting it, but the ultimate arbiter there is Bangladesh (who still say she isn't one of their citizens).
Also as I said above, if we'd rescinded her citizenship when she joined IS that would be one thing - but we didn't; we rescinded it when she'd left IS.
Section 5 of the Citizenship Act 1951 states that, a person born outside Bangladesh ‘shall be a citizen of Bangladesh by descent’ if either of his or her parents is a citizen of Bangladesh at the time of his or her birth.
Therefore, a person is automatically a citizen of Bangladesh at birth if either of his or her parents is a Bangladeshi citizen by birth (i.e. was born in Bangladesh).
So she's not stateless as she is eligible for Bangladeshi citizenship, until the age of 21. Worth a read. Complicated but the law generally comes down on the side of the UK here.
Shamima Begum may be a Bangladeshi Citizen After All
In 2015, Ms Shamima Begum, then a 15-year-old British citizen living in London, travelled to Syria to join the so-called Islamic State. Her fate was unknown until recently when Ms Begum was discovered in a refugee camp in Syria. On 19 February 2019, the British Home Office in a letter delivered...
www.ejiltalk.org