I saw your other comment that you meant interface. But an example of a language that went without a feature people thought the language desperately needed was Go with generics. They only added them more than ten years later, when they figured out the best way to implement them.

It might be the same with gleam, with first version in 2019 and 1.0 in 2024. The language authors might think they are either uneeded and lead to anti patterns, or are waiting to see the best way to implement them.