The current state of research excludes none of the two cases. The occurrence of some small cycles has been actually ruled out. You can easily find the relevant references in the Wikipedia page devoted to Collatz conjecture (the same that you linked), at the section "Cycles".
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was misleaded by the initial formulation "the existence of some cycles has been ruled out" may sound as "there's no cycle"
YCor
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Francesco Polizzi
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