I use Goodreads for books and Letterboxd for movies, but only a shoebox for concert ticket stubs.
Bachtrack is classical-focused and lets you log concerts from their catalog, which appears to only cover major cities. Concert Archives has an attractive set of features, but I think classical music has additional dimensions to track (composers, conductors, soloists) and benefits from a more focused app.
Concert Liszt lets you record concerts you've attended with the composers and pieces you heard at each one.
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