Who understands copyright best? IMSLP would definitely be shortlisted. They have an excellent article explaining general how copyright works:
A work that is in the public domain is not protected by copyright and can be freely used for any purpose. Which works are in the public domain varies from country to country.
For example, [in Canada] if a copyright lasts for 70 years after an author’s death, and the author who died on March 14, 1955, then the term lasts until December 31, 2025 and the work is in the public domain from 2026 onwards.
And—let’s rejoycify that some composers are “Coming Soon”: Shostakovich (2026), Sibelius (2028), Poulenc (2034), Ravel’s Piano Concerto in G Major (2026) and Prokofiev’s Romeo and Juliet (2030).