Optional reagents are new types of reagents that can change the item level and the secondary stats of the items you craft. You can probably guess from its name, but it isn't required to fill the Optional Reagent slot when you create something, you can craft an item with no optional reagents or you can fill as many Optional Reagents slots as you prefer.
1. Secondary Stat Changing
These Optional reagents basically let you choose which secondary stats you want on an item, instead of just getting two random stats. (you can't use two from the same stat)
All 4 of these below are crafted by Scribes.
Missive of Critical Strike
Missive of Haste
Missive of Mastery
Missive of Versatility2. Item Level Changing
These reagents change the item level of a crafted armor or weapon. (You can actually downgrade an item's item level and its level requirement if you use a lower level optional reagent)
2.1. Shadowlands Items
These 3 optional reagents below can only be used to craft items from Shadowlands. All 8 manufacturing professions can craft these.
Novice Crafter's Mark - Set the Item Level to 87 and the Required Level to 50.
Crafter's Mark I - Set the Item Level to 117 and the Required Level to 55.
Crafter's Mark II - Set the Item Level to 168 and the Required Level to 60
2.2. Older Expansions
The five types of Relic of the Past below can be used on items from older expansions, and cannot be used to craft items from Shadowlands.
Only Tailors, Blacksmiths, and Leatherworkers can craft these.
Relic of the Past I - Set the Item Level to 19 and the Required Level to 14
Relic of the Past II - Set the Item Level to 28 and the Required Level to 23.
Relic of the Past III - Set the Item Level to 39 and the Required Level to 32.
Relic of the Past IV - Set the Item Level to 48 and the Required Level to 41.
Relic of the Past V - Set the Item Level to 54 and the Required Level to 47.
All 5 can be crafted with 24 different materials. (different materials from each expansion for each profession). So, for example, you can craft Relic of the Past I with 8 different cloths. The 8 recipes below all make the same item, the different versions were added so it won't matter where you level with your character, you can always make useful gear by using these.
- Relic of the Past I - 5 x
Linen Cloth, 1 x 
Penumbra Thread - Relic of the Past I - 5 x
Tidespray Linen, 1 x Penumbra Thread - Relic of the Past I - 5 x
Sumptuous Fur, 1 x Penumbra Thread - Relic of the Past I - 5 x
Netherweave Cloth, 1 x Penumbra Thread - Relic of the Past I - 5 x
Shal'dorei Silk, 1 x Penumbra Thread - Relic of the Past I - 5 x
Windwool Cloth, 1 x Penumbra Thread - Relic of the Past I - 5 x
Embersilk Cloth, 1 x Penumbra Thread
Where to learn Relic of the Past recipes?
You can learn the recipes from each expansion's profession trainer. (you will automatically learn the new Legion and Draenor Recipes).
You can find the trainer's location in each of my profession leveling guides if you are having trouble finding them. You can also just walk up to a guard in major cities then ask where the profession trainer is. Asking the guard will place a red marker on your map at the trainer's location.