Taking the first element in a list of associations

You can use the fact that Part ([[…]]) preserves the keys when you supply a list of indices to extract:

z[[All, {1}]]
(* {<|{"a", "b"} -> 1|>, <|{"g", "h"} -> 4|>, <|{"k", "l"} -> 
   6|>, <|{"s", "t"} -> 10|>} *)

(* without the list *)
z[[All, 1]]
(* {1, 4, 6, 10} *)

Alternatively you could use Take:

Map[Take[#, 1] &, z]
(* {<|{"a", "b"} -> 1|>, <|{"g", "h"} -> 4|>, <|{"k", "l"} -> 
   6|>, <|{"s", "t"} -> 10|>} *)

Other possibilites:

Map[#[[{1}]] &, z]
(* {<|{"a", "b"} -> 1|>, <|{"g", "h"} -> 4|>, <|{"k", "l"} -> 
   6|>, <|{"s", "t"} -> 10|>} *)

z[[All, ;; 1]]
(* {<|{"a", "b"} -> 1|>, <|{"g", "h"} -> 4|>, <|{"k", "l"} -> 
   6|>, <|{"s", "t"} -> 10|>} *)

In essence, anything that could in principle return multiple elements will preserve the keys (or, more generally, will preserve that level of the nested expression)

For completeness sake, a few more:

Extract[z, {All, {1}}]
(* {<|{"a", "b"} -> 1|>, <|{"g", "h"} -> 4|>, <|{"k", "l"} -> 
   6|>, <|{"s", "t"} -> 10|>} *)

Cases[z, KeyValuePattern[{x_ -> y_}] :> x -> y]
(*{{"a", "b"} -> 1, {"g", "h"} -> 4, {"k", "l"} -> 6, {"s", "t"} -> 10}*)