You've seen research note boxes used with maintenance categories, as you have shown the list, above. You may use them freely if they already exist.
If you click on your above link for Needs Birth Record, you will be taken to [[Category: Needs Birth Record]]. Notice that there are both subcategories and profiles attached. Click on the subcategory for German Roots Project Needs Birth Record. Notice in that category there is an explanation for using the template: {{German Roots|Needs=Birth Record}}
By adding the qualifier "|Needs Birth Record" to the German Roots template, it places the profile into the subcategory "German Roots Project Needs Birth Record" automatically.
STICKERS, I believe, are not currently doing this and, I do not believe they should do so. Simply add the category manually. As you explained, stickers are to highlight a fact or an achievement.
If you are working on a non-project related profile, you may add maintenance categories freely. There are many:
https://www.wikitree.com/wiki/Category:Maintenance_Categories