A composer enters into the Public Domain (=free to publish, perform without license, and arrange without license) generally 70 years after composer's death.
This year we have, among others, these composers "free": Kurt Weill and Charles Koechlin.
Here are composers that will soon "become free":
2021: Arnold Schoenberg
2023: Sergei Prokofiev
2024: Charles Ives, Nikolai Oboukhov
2025: Arthur Honegger, George Enescu
2026: Reinhold Gliere, Gustave Charpentier
2027: Jean Sibelius, Erich Korngold
2028: Ralph Vaughan Williams, Florent Schmitt
2029: Bohuslav Martinu, George Antheil, Heitor Villa Lobos, Ernest Bloch
However, any editorial edition has "new" copyright attached to it, so called publisher's edition copyright. Indeed, a new edition of a Beethoven sonata can be copyrighted, despite the fact that Beethoven "is free" - only because the editor has his/her own rights. (Think how much effort/time/knowledge John Ruggero has put in his edition!!)
Usually, IMSLP has a very strict and systematic database and there you can easily obtain free scores without worrying.