From the Ovation Rewards website:

"We have discontinued the sale of Ovation Rewards Certificates starting in 2025.

Certificates already issued will continue to be honoured for redemption without expiry."

I found the note when I went to the website to show the hotel I was staying at what was included with the certificates. I don't know if there is any correlation but the last two times I have tried to use Ovation Rewards certificates I have ran into multiple problems (see my post under Sofitel New York, but also had problems at the Fairmont Queen Elizabeth in Montreal this past weekend). This program was continued when Accor acquired Fairmont.