The bill is still pending.
Several undecided issues remain.
We left that issue pending.
The case remains unresolved even now.
保留 implies a decision is deliberately being held back or postponed, whereas 未決 simply states that it has not yet been settled.
未決 is a more formal/legal term for something pending a decision (e.g. court cases), while 未解決 is broader and covers any unsolved problem, mystery, or issue.
Compound of 未 (mi, 'not yet') and 決 (ketsu, 'decide'). The spelling and meaning are transparently derived from the kanji components.