[ESMA 70-1861941480-56 MiFIR data reporting Q&A, Q&A 5.2]

For bonds that were not paid out on the originally intended maturity date (and therefore have not expired, because, for example, they defaulted), the maturity date should be set to 9999-12-31 (in accordance with the ISO format, like for perpetual debt financial instruments) until they are finally paid out, in which case the maturity date should be updated with the actual date of redemption”.