Delta’s strategy for “hot” markets during the “hot season(s)” (and Europe in the summer and fall are certainly “hot”), is to zero out the cheaper fare buckets (Z and sometimes even I) far in advance (it is now further in advance than Pre-Covid because many leisure travellers are buying D1 in advance and leisure flyers usually book way in advance). What I have noticed is they will release Z inventory closer to the travel date. They are betting that people rather book in advance and be “secure” with a confirmed D1 seat to Europe during spring/summer/fall than wait till Z opens up. So basically it becomes a game to see how long you can wait (and deal with the uncertainty).