Answer
COVID certificates issued before 1 September 2023 can still be used for travel if need be, as long as they comply with the destination country’s entry regulations and their QR code has technically not yet expired. The QR code for each COVID certificate is technically valid for two years.
Alongside the COVID certificate, alternative evidence of vaccination or having had COVID and recovered is usually accepted. In general, the evidence required is a sovereign matter to be decided by each individual country. Always look on the relevant authorities’ websites for your transit and destination countries to clarify what evidence they accept.