Using pure functions to assign a value to variable depending on whether it has been defined already

You say that you want to reproduce the behavior of If[Not[ValueQ[a]], a = 2020]. This will assign a value and return it if a is not assigned, or return Null if a already has a value.

I wonder if the following function would work for you then:

ClearAll[condassign]
condassign[variable_Symbol, value_: 2020] := (variable = value)
condassign[variable_?NumericQ, value_: 2020] := Null

The value itself can be given as a second argument, but if omitted it is assumed to be 2020.

Here are two examples:

a =.                            (* Clear any value in a              *)
condassign[a]                   (* Out: 2020 because a had not value *)
Print@condassign[a]             (* Out: Null now that a has a value  *)

The Print expression above is only to show the Null return value, which otherwise would not appear explicitly.

Clear[a]
If[Head[a] === Symbol, a = 2020]
a                                       (* 2020 *)

a=10;
If[Head[a] === Symbol, a = 2020]
a                                        (* 10 *)