As you may be aware that seasonal weapons have a craftable option, only if you bought the seasons or the annual pass and once you have obtained enough patterns for them. The weapons from the season pass track rewards (2 each season) given to free players were not subject to that restriction, there was no way to obtain these weapons to craft at that point in time for these players as it was exclusive to paid players.

However, once the year ended and a new Destiny year started (Lightfall - Season 20), the previous year's weapons still could not be crafted unless you owned the old season passes. And since Banshee and Xur can sell these weapons, and you can use a Harmonizer to obtain the pattern, you won't be able to craft them still as they are locked behind the old season passes and is no longer possible to buy.

Bungie deployed a fix this week (27 June 2023) when a community post gained traction about Bump in the Night not being able to be crafted. But this was not exclusive to just that one gun, and other guns suffered the restriction too and was not made aware of. With this week's patch, some guns from the old seasons got their restriction removed and was not mentioned in the patch notes. Which is seen as a positive but some guns were left behind and still cannot be crafted. Please fix this issue and allow the remaining old seasonal weapons to become craftable for everyone once they have obtained enough of the patterns. As using your Harmonizer on these weapons would be a waste and we will have people posting about this issue in the near future.