Can the Tax Office still reassess after 5 years?
Normally no — 5 years is the standard limit (AWR art. 16). Exception: concealed foreign income → 12 years. Outside that: object on time-bar grounds.
AWR art. 16 sets reassessment allowed within 5 years after the tax year — extended to 12 years for concealed foreign income. Outside that, the reassessment is in principle void and you should OBJECT within 6 weeks of date stamp. Reassessment is also ONLY allowed on a "new fact" (information the inspector lacked and reasonably needn't have had) or "bad faith" (intent or gross negligence of taxpayer). Standard "we overlooked it" errors by the inspector cannot lead to reassessment — that's "official negligence" and shouldn't fall on you. When in doubt: in your objection explicitly demand justification of both the "new fact" and the time-bar.