The Peacekeeper shotgun is also slightly weaker, but it hasn't been nerfed directly. Instead, the Shotgun Bolt attachments don't increase the rate of fire as significantly as they did before, but only when paired with the Peacekeeper specifically (leaving other shotguns unaffected). Perhaps the biggest hit on both guns, though, is that they're now harder to find, while energy weapons and ammo boxes are no longer quite so rare. Respawn Entertainment explained in a Reddit post (where you'll find further details on the changes) that it decided to tweak the strongest weapons and leave everything else as it is to preserve the game's "power curve."
In other words, guns like the P2020 pistol and Mozambique shotgun are supposed to be weaker, and something you hope to discard ASAP. In the same way you're always after armor, equipment and attachment upgrades, maintaining a gun tier list of sorts (because everyone has their preferences) encourages players to move around the map and hunt for kills in order to improve their loadout. Respawn also says its general approach to balance is to make minor, impactful changes that have been thoroughly playtested, giving players ample opportunity to master weapons and characters without a balance patch meddling with the metagame every week.
While there are no changes to the legends themselves in this balance patch, Respawn has confirmed a few minor tweaks to character abilities will be introduced alongside the Season 1 Battle Pass. More importantly, though, the same update will attempt to address the issue of the larger legends having unfairly bloated hitboxes. As it stands, the bigger the legend, the easier they are to shoot. The game is designed that way, but players have complained that the benefits of using the larger characters don't feel like they make up for the fact they're bigger targets. Respawn's going to start by making small adjustments to the size of hitboxes, but may consider other changes like reducing incoming damage or increasing the power of abilities.
Again, Respawn doesn't want to drastically alter the feel of the game with any one update, which seems sensible when you're currently the hottest battle royale title that's attract over 50 million players just a month after release.