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When evaluatingf[x_:x0] := x, x0 will be evaluated, regardless of the HoldAll attribute of SetDelayed.

Why the second argument of Optional is evaluated in a non-standard manner?

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From the "Details" of the docs for HoldPattern

The left-hand sides of rules are usually evaluated, as are parts of the left-hand sides of assignments. You can use HoldPattern to stop any part from being evaluated.

Application:

ClearAll[f];
f[HoldPattern[x_: x0]] := x;

x0 = 7;

f[5]
f[]
(*
  5
  7
*)

Block[{x0 = 3},
 f[]
 ]
(*  3  *)
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