Was just on the Sapphire princess in September and was able to have dairy alternative milk everywhere on the ship. The only issue is some locations may not have 1 specific type (tried to ask for oat milk in the MDR but they didn't have it, alternated to soy).
Could not locate the dairy alternative milk at the buffet for self service but the buffet waitstaff were able to bring some really quickly when politely requested.
The OceanReady App if you order coffee through it gives option to substitute the milk to an alternative of choice.