I would be highly surprised if Lexus didn't take care of such a basic, required-by-law safety feature, even after the warranty. A seatbelt should last forever.
Basically seatbelts and related components are covered for 6yrs/70,000 miles. But some dealers replace it (seatbelts) under Goodwill Warranty. So it's best to ask your dealer or Lexus about coverage.
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