The case that you mentioned isn't an example of what you're talking about.
One thing that immediately comes to mind is the Shamima Begum case.
She fled her London home to join the Islamic state but now she wants
to come back to the UK (after realizing), but UK's Home Office revoked
her citizenship, claiming that she could claim Bangladesh citizenship
by descent even though she isn't a citizen of Bangladesh at the time
of revocation.
No, they are claiming that Shamima Begum is a citizen of Bangladesh at the time of revocation.
According to section 5 of Bangladesh's Citizenship Act 1951, a child born abroad to a Bangladeshi citizen father is automatically ("shall be") a Bangladeshi citizen by descent at birth. (Mothers were allowed to pass on citizenship after 2009, but that was after Begum was born.) Note that registration at a Bangladeshi consulate within 1 year of birth is only necessary in the case where the father is a Bangladeshi citizen by descent. I believe Begum's father was a Bangladeshi citizen otherwise by descent, in which case no registration or other action is necessary for her to be a Bangladeshi citizen at birth. It doesn't matter that she has never been to Bangladesh nor does it matter that she never claimed to be a Bangladeshi citizen.
There were two men of Bangladeshi descent in a separate case who successfully fought their revocation of British citizenship, but the difference between their cases and Begum's case was that they were over 21, which she was under 21 at the time of revocation. Section 14 of Bangladeshi's Citizenship Act provides that someone with dual citizenship automatically loses Bangladeshi citizenship if they don't renounce their other citizenship, but this provision doesn't apply to those under 21. So these two men had Bangladeshi citizenship too, while they were under 21, but they lost it when they turned 21, before their supposed revocation of British citizenship, whereas for Begum, she hadn't lost Bangladeshi citizenship at the time of the revocation of her British citizenship, because she hadn't turned 21.
(Perhaps you got the idea of "claiming" of citizenship from some report that one can "claim" Bangladeshi citizenship by descent while under 21, and these men failed to claim it, but Begum can still "claim" it. But if you read the text of the law, that is clearly not the case. For a child born to a father who was a Bangladeshi citizen otherwise than by descent, there is no "claim" of citizenship -- it is automatic and involuntary at birth.)
As to your question, there are no universal restrictions to how a country can grant or take away citizenship. There is the 1961 Convention on the Reduction of Statelessness, which countries may voluntarily join, but only a minority of countries of the world are party to the convention. Article 8 of the Convention does prohibit countries that are party to the Convention from depriving someone's citizenship if it would "render him stateless", though there are several exceptions including if the citizenship was obtained through fraud or misrepresentation. The language seems to require that the person already have another citizenship, not just have the ability to acquire one, though I am not sure how much leeway countries have to interpret this.
In the case of the UK, it is a party to the Convention, and it has largely implemented the provisions of the Convention in its domestic law. With respect to deprivation of citizenship, section 40 subsection (4) of the British Nationality Act 1981 prohibits a deprivation order if the Secretary "is satisfied that the order would make a person stateless." (Subsection (4A) has a looser restriction where British citizens by naturalization can be deprived of citizenship if the Secretary believes that the person is able to become the national of another country. I am not sure whether this is compatible with the Convention. In any case, this is not relevant to Begum's case as she was not a British citizen by naturalization.) So if the UK were to try to deprive citizenship of a British citizen otherwise than by naturalization like Begum, not on the basis that the person already has another citizenship but on the basis that they are "eligible" to "claim" one (which as I described above I do not believe is the case for Begum; I am talking hypothetically if such a case were to arise), that can already be challenged as a violation of British law, in British courts, without considering the UK's obligations under the Convention.
If it's another country that's a party to the Convention, but their law expressly allows deprivation of citizenship for being "eligible" to acquire another citizenship even though the person doesn't have one (including, perhaps, British citizens by naturalization deprived citizenship under section 40(4A)), and a person in that situation is deprived of citizenship, they don't really have any recourse. A private party cannot "sue" a country over any violations of the Convention in an international court.
