In Destiny 2 Shadowkeep, the grenade launchers are not as effective as they are now. Nerfs elsewhere are significantly limiting their DPS.
This is currently what makes grenade launchers so special: At the launch of Destiny 2 in 2017, grenade launchers were indeed an interesting, new weapon class – but pretty useless. At that time, they could not compete with rocket launchers or other power weapons, the Lion’s Roar was even considered the worst exotic weapon overall.
By the season of the Wanderer, that changed. At the beginning of 2019, the weapon class received a massive damage buff, making grenade launchers a must-have in the inventory. They are absolute boss destroyers and deal enormous damage per second in PvE.
With Shadowkeep, their power is indirectly restricted.
Grenade launchers require laborious reloading
This change nerfs grenade launchers: With Shadowkeep, Bungie is removing the auto-reload, the automatic reloading mechanic. Specifically, the Lunafacture boots in conjunction with Rift/Well of Radiance and the Barricade of Gunfights lose this property and instead only grant an increased reload speed.
The auto-reload was always an essential part of nearly all challenging PvE activities and boss fights. You would position yourself in the effective range and shoot everything at the enemy without reloading, which significantly increased the damage over time.

Why does this hit grenade launchers hard? While grenade launchers now have an enormous damage output, they are hindered by the long reload time and the low ammunition capacity. With the auto-reload mechanic, the drawbacks were circumvented, allowing for constant fire on the boss.
If auto-reload goes away, it hits grenade launchers particularly hard. Most of them have only 1 to 6 shots in the magazine, after which reloading takes several seconds. This greatly reduces damage over time and thus the effectiveness of grenade launchers.
Thus, grenade launchers will lose a significant amount of their power when auto-reload disappears. How useful they will still be in Shadowkeep remains to be seen. Possibly the “significant increase in reload speed” that replaces the auto-reload will be rather high, making the nerf not so severe. But the time of dominance for grenade launchers should be numbered.

“No nerfs for grenade launchers”
What does Bungie say about this? Last week, Bungie rolled out an update that made the quests for the Summit and for Wendigo easier. These are two top-tier grenade launchers that are among the best boss-killers in Destiny 2.
When asked whether these grenade launchers will still be worthwhile with Shadowkeep, community manager dmg04 assured that grenade launchers will not receive nerfs – including Summit and Wendigo.
Ultimately, this statement is correct – grenade launchers will not be directly nerfed. But their effectiveness will still be restricted indirectly.