They are not given away to the public, although they can be found for sale on the secondary market
Then, around 20172020, it jumped
After the first set of cards in Legends and a few other sprinklings, many of which established that gold cards would be consistently more powerful than their competition, gold came into its own in Invasion block
This means that you can fit split second cards into virtually any deck , either as a backup for a spell that you need, or as an alternative that your opponents can't deal with
4 cards are of rarity Rare or higher in every pack
If you do cast it for its harmonize cost, you must exile the harmonize spell as soon as it leaves the stack, no matter how it leaves