English is a living language. The suffix -hood is available for the making of new words; so, even if 'sovereignhood' hasn't appeared in any dictionary yet (I don't know - I haven't checked), there's no reason to think that the word shouldn't exist.
However, there is already an abstract noun derived from "sovereign" - sovereignty. But that has particular significance related to the adjective "sovereign"; The Republic of Argentina and the UK disagree about the sovereignty of the Malvinas/Falkland Islands. So anyone who supports the use of the word 'sovereignhood' should be sure that it doesn't overlap with this meaning. If it means 'being a sovereign', there's no such overlap.
However, there's already the word "kingship".... Quite possibly, supporters of the new word have chosen to use it because it means kingOrQueenShip.
In summary: whether or not a new word appears in a dictionary, if it's clear and useful it will be used - and if it's used it will appear in a dictionary some time soon.
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