Because the historians among us like this sort of thing as do I.
The Leipzig Gewandhaus has a searchable one here (https://www.gewandhausorchester.de/en/archiv) ;
the Vienna Symphony a more limited (1900-present only by date, but also searchable by composer) here (https://www.wienersymphoniker.at/de/archiv/suche) ;
the Vienna Philharmonic's is here (https://www.wienerphilharmoniker.at/converts/archive).
IIRC the New York Philharmonic has a huge archive too which I may edit in later. And others, too...
(The relevance to this site is somewhat more limited to for the Vienna Symphony, but even they performed a fair amount of unsung music between 1900 and 1915, eg, Pick-Mangiagalli's ballade symphonique for piano and orchestra Op.12 and Allegro di Concerto Op.10 + 3 Miniatures in a concert on February 7 1910..)
Thank you. This is exactly the sort of thing I like to explore. The "Staatliches Institut für Musikforschung" (SIM) in Berlin has the "Archiv des Konzertlebens" (https://www.programmregister.de/LNG=EN/?COOKIE=U999,K999,D1.79,E163cdc7e-e,I0,B9994++++++,SY,A,H12-15,,17-23,,30,,73-78,,88-90,NGAST,R185.230.125.21,FN) and the guide to concerts performed in the past "Konzertführer Berlin Brandenburg 1920-2012" (http://digital.sim.spk-berlin.de/viewer/browse/ak.kfbb/-/1/-/-/).
Thank you for these splendid links - much appreciated.
Thank you very much for this. Very useful.