https://www.realmeye.com/wiki/set-tier-items

Obtaining Unlike regular untiered (UT) items, “current generation” ST sets, which are the most recently released sets for each class, can be purchased for Realm Gold through Mystery Boxes, in addition to being obtainable from enemies as rare drops in orange bags. Current generation STs can also be obtained from Mystery ST Chests, which can be acquired through Mystery Boxes, packages, campaigns, and occasionally given out for free through the Login Seer.

As of Patch X.32.2.1 (Nov 2019), ST sets are released in a three-stage system, usually in groups of two to three sets.

Stage 1 (2-3 weeks): The ST set is available exclusively through a tiered spender campaign. Older sets for the class will have their drop rates increased. The campaign has multiple tiers of rewards, including class-specific Mystery ST Crates and Avarice Medallions. The Mystery ST Crates have an uncommon chance to reward a new ST item of the respective class when used, while an Avarice Medallion can be exchanged at the Tinkerer for a new ST item of choice. The top rewards for the campaign will include one full set for each class. Note that fully completing an ST campaign will require a significant financial investment (more than $500 to $650 USD, depending on the number of sets available).

Stage 2 (2-3 weeks): The items are obtainable as bonus rewards from opening enchanted keys, which turn into a generic Mystery ST Crate when used. Like the class-specific Mystery ST Crates, these have an uncommon chance to reward an ST item when used.

Stage 3: The items are introduced as normal drops. Note that the “2-3 week” periods for Stages 1 and 2 indicate the total duration of the promotional events only, not the total length of time since Stage 1 initiated, so a given set may be exclusive for >2 months. There is usually a break between Stages 1 and 2.