Destiny 2 is just a few weeks away from the next season, which will introduce more Adept weapons. The current ones will not be removed but they will be in a waiting line.
Palindrome is possibly the most popular among the weapons exclusive to Nightfall strikes, and currently on rotation so get farming if you want it, but it will be semi-retired when Season of the Chosen ends.
Unlike sunsetting and vaulting, this type of retiring is not actually bad. The reason Palindrome, Shadow Price and The Swarm are temporarily going away is to give space to new Nightfall weapons that will be introduced in Season 14.
Bungie didn't offer much info on these new weapons other than saying there will be a total of three of them and they will start their own rotation on May 11, 2021, when S14 hits.
The trio that we currently have will be retired for only the first half of Season 14 so keep an eye out for these drops in the latter half, provided you didn't farm up the rolls you wanted already.
Grandmaster Nightfalls have their own staggered start schedule so keep in mind that the Adept versions of these new weapons will not be available before that happens.
Considering that the Palindrome will not be dropping for months after the next weekly reset, we highly suggest you dive into the Nightfalls if you're still looking for that High-Impact Reserves roll.