A unit pushes in by itself to forbidden for 5, then wins combat, after showdown sacrifice is played, however forbidden says you have -2 when defending. After showdown is it considered defending?

No, the unit is no longer considered defending.

According to Rule 459.2.b.4 and FAQ #2171, a unit is only considered a "defender" when it possesses the Defender designation during an active combat. Per the Showdown Flow guidelines, once a combat showdown concludes, all designations (including "attacker" and "defender") are removed from units.

As a result, any effects that apply "while defending" (such as a penalty for being a defender) cease to function immediately after the combat ends, regardless of whether a sacrifice or other spell is played afterward.

Since I couldn't find a card exactly named "Forbidden," I based this ruling on the general mechanics of the Defender designation. If you can provide the exact text or name of the card, I can provide a more specific analysis!