I'd say a large percentage of pieces are unjustly neglected, because as the decades passed, our ability and capacity to understand music beyond what we already know, went away.
One could use Anton Rubinstein's Piano Concerto No. 5 in E-flat, Op. 94, as an example of this.
Ferruccio Busoni understood it well enough to champion it and play it frequently enough; Josef Lhevinne played it for his NYC debut in 1919. But ultimately, after Busoni and Lhevinne were gone, this piece went away from the fringes of the repertoire as well. Rubinstein's much more popular Piano Concerto No. 4 in D minor, Op. 70, did not survive much longer either.
Usually interest is revived in a piece when someone famous takes it up; take one of my favorite concerti, the Edward MacDowell Piano Concerto No. 2 in D minor, Op. 23, as an example of this phenomenon. That particular piece stayed in the repertoire 70 years longer than it would've, in part thanks to Van Cliburn and Andrè Watts. Cliburn, particularly, had a fondness for this concerto and kept it in his active repertoire reportedly as late as 1999.
Even in music by standard composers, you see that some pieces are recognized as greater than others: Ludwig van Beethoven's Emperor concerto always reigns supreme versus the same composer's own Op. 61a transcription of the Violin Concerto for piano & orchestra. Why? Because people don't take the time to understand the Op. 61a concerto of Beethoven.
So, this is a trend. And it's only going to continue with COVID-19.