Question: Since what year has the anthem which gradually supplanted the Sturmlied as the de facto anthem of the SA been banned in Germany?

Context paragraphs:
Act on National Flag and Anthem: The Act on National Flag and Anthem (国旗及び国歌に関する法律 , Kokki Oyobi Kokka ni Kansuru Hōritsu ) (abbreviated as 国旗国歌法 ) is a law that formally established Japan's national flag and anthem.  Before its ratification on August 13, 1999, there was no official flag or anthem for Japan.  The "nisshōki" (日章旗 ) flag, commonly referred to as the "hinomaru" (日の丸 ) , had represented Japan unofficially since 1870; "Kimigayo" (君が代 ) had been used as Japan's "de facto" anthem since 1880.

Sønner av Norge: Sønner av Norge (originally "Sønner af Norge", literally "Sons of Norway") is the common title of the anthem Norsk Nationalsang (literally, "Norwegian National Anthem"), which was the "de facto" national anthem of Norway from 1820 until the early 20th century.  From the mid-1860s, "Ja, vi elsker dette landet" gradually came to occupy the unofficial position as national anthem, but was used alongside "Sønner af Norge" until the early 20th century, with "Sønner af Norge" being preferred in official situations.

Mawtini: Mawtini (Arabic: موطني‎ ‎ “My Homeland”) is an Arabic anthem which since 2004 has served as the national anthem of Iraq, and was previously the de facto anthem of Palestine.

Horst-Wessel-Lied: "Horst-Wessel-Lied " (English: "The Horst Wessel Song" ; ] ), also known by its opening words, "Die Fahne hoch " ("The Flag on High"), was used as the anthem of the Nazi Party from 1930 to 1945.  From 1933 to 1945 the Nazis made it the national anthem of Germany, along with the first stanza of the "Deutschlandlied".  Since 1991, the anthem has been banned in Germany.

Cleveland Rocks: "Cleveland Rocks" is a rock song by Ian Hunter from his 1979 album "You're Never Alone with a Schizophrenic".  The song is seen as a de facto anthem in Cleveland, Ohio.  The song was played every Friday at 6:00 PM on Cleveland radio station WMMS beginning in 1979 and is used as a victory song for the city's sports teams.  In recognition of "Cleveland Rocks", Hunter was given the key to the city by Cleveland mayor Dennis Kucinich on June 19, 1979.

O Canada: "O Canada" (French: "Ô Canada" ) is the national anthem of Canada.  The song was originally commissioned by Lieutenant Governor of Quebec Théodore Robitaille for the 1880 Saint-Jean-Baptiste Day ceremony; Calixa Lavallée composed the music, after which, words were written by the poet and judge Sir Adolphe-Basile Routhier.  The lyrics were originally in French; an English version was created in 1906.  Robert Stanley Weir wrote in 1908 another English version, which is the official and most popular version, one that is not a literal translation of the French.  Weir's lyrics have been revised twice, taking their present form in 1980, but the French lyrics remain unaltered.  "O Canada" had served as a "de facto" national anthem since 1939, officially becoming Canada's national anthem in 1980 when the Act of Parliament making it so received royal assent and became effective on July 1 as part of that year's Dominion Day (now known as Canada Day) celebrations.

Giovinezza: "Giovinezza" (] ; Italian for "youth") is the official hymn of the Italian National Fascist Party, regime, and army, and was the unofficial national anthem of Italy between 1924 and 1943.  Although often sung with the official national anthem Marcia Reale, some sources consider Giovinezza to have supplanted the Royal March as the "de facto" national anthem ("Inno della Patria") of Italy, to the dismay of Victor Emmanuel III of Italy—a powerful symbol of the diarchy between the King and Mussolini.  It was subsequently the official anthem of the Italian Social Republic.

The Land (song): The Land is a protest song, traditionally sung by the Georgist movement in Britain in pursuit and promotion of land value taxation.  Until the late 1970s it was sung at the end of each year's Liberal Assembly and was the party anthem of the Liberal Party until that party merged with the SDP to form the Liberal Democrats.  To this day it remains the "de facto" anthem of the Liberal Democrats, is sung as the first song of the Liberal Democrats Glee Club and is the party anthem of the continuity Liberal Party.  During the chorus, the phrase 'ballot in our hand' is accompanied by the collective waving of any paper to hand (usually a Liberator song book) by the audience.

Aiaaira: Aiaaira (Abkhaz: «Аиааира» , "Victory") is the de facto anthem of the partially recognized state of Abkhazia.  It was adopted in 1992.  The lyrics were written by poet Gennady Alamia, and the music composed by Valery Chkadua.

Sturmlied: The Sturmlied ("Storm Song" or "Assault Song") was the de facto anthem of the SA until it was gradually supplanted by the Horst-Wessel-Lied.

Supporting paragraphs: Sturmlied Horst-Wessel-Lied
Answer: 1991