You can no longer buy them to use for upgrades. Instead, any leftover stacks in your inventory can only be "sold" to at the Tower for small amounts of Glimmer. Historical Context and Usage
: Described as filaments of helium-3 fusion fuel, they were canonically gathered from lunar regolith and used to power everything from individual fusion rifles to the Last City itself. helium filaments destiny buy
In their prime, Helium Filaments were more than just a currency; they were the "fuel" for progression. You can no longer buy them to use for upgrades