Materials from the planets that will be vaulted in Destiny 2 Beyond Light will be useless soon so it might be a good idea to use them up at vendors while you can.
This Week At Bungie revealed the currencies from Mercury, Mars, Titan and Io will not magically disappear from your inventory when Season of the Hunt starts. They will, however, lose their worth in the season after.
During Season 12, which starts in November 2020, players will be able to use these planetary materials as usual, except for the vendors on said planets.
Furthermore, Spider will be accepting these materials as well, in exchange for a small amount of Glimmer. Therefore, if you're looking to use these materials for something useful, make sure it's for Cores from Banshee or even items from planetary vendors before they are gone.
Glimmer can be obtained fairly easy from many sources and we all end up at the cap anyway so trading these materials for this currency is likely the worst possible option you can get. However, sharding the items you get from planetary vendors in exchange for the mats will grant you legendary shards and possibly masterwork materials, on top of glimmer you get from deconstructing the shaders that often come in their engrams.
In possibly the saddest part of the new season, the Expired Ramen Coupon will be gone as well.