So it was available as a bonus before? I wonder if I can get a copy if I request it via Support
No, it was being sold as a separate title.
'The main difference between Myst and Myst: Masterpiece Edition is that the former uses an 8-bit color palette, while the latter uses a 24-bit color palette. This is a noticeable difference if you play the two games.'
The 25th Anniversary box set does come with MYST Classic ....but you're really not missing anything ..... and YES the version you played more than likely was 'Classic'
For your reading pleasure: https://gaming.stackexchange.com/questions/15098/myst-masterpiece-edition-vs-realmyst