I know a lot was improved in the Masterpiece Edition, but I'm positive the version I played back in the day was the original release. I wish GOG included it as an extra at least.

It was pulled down on request from the makers, due to them feeling it aged and unneeded.

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

I'm pretty sure the Myst that's available for mobile is the original.

Nope, that's RealMyst, based on the 2000 version.