“Best by” often means “will lose taste over time and taste worse by” when it comes to chemical products like these.

If this stuff contains lots of sugar or no natural compounds at all, give it a try, I guess. Especially if they’re sealed in a box. Trust your senses, don’t try too much at once if you don’t know if it’s still good, and don’t swallow until you’ve verified there’s no taste of rot or other grossness. If you want more safety, have a bottle of vodka ready to rinse your mouth with in case you do notice a bad taste.

If these powders are completely dry and stored airtight (and the packaging isn’t damaged), you could probably store them for literal decades without a problem. If there’s stored in uncoated cardboard (no sealing lining), maybe treat them as mold infested, even before their expiration date. The way they’re stored makes all the difference.