The museum has 60 donations. Where is the Rusty Key?
Count objects placed in the museum, not artifacts appearing in your personal collection. In the base game the reward usually arrives at your farmhouse rather than in the museum rewards window.
Audit the display rather than the collection tab
Gunther needs 60 different items actually donated, including minerals as well as artifacts. Finding a mineral, shipping it or keeping it in a chest does not fill a display spot. Compare your spare items with the museum and bring any still eligible for donation. The early forest cutscene about a strange sewer noise is a clue for the location, not the event that hands you a key.
Start the next day and check the host
In single-player, sleep after the qualifying donation and leave through the farmhouse door the next morning to meet Gunther. Look in Special Items & Powers in version 1.6 rather than emptying your backpack for an inventory slot. On a multiplayer farm have the host speak to Gunther as well. If the save uses Stardew Valley Expanded, check that mod’s instructions separately; it changes the acquisition route.
Use either unlocked entrance
For the town entrance, inspect the metal sewer cover in southern Pelican Town. For the forest entrance, go to the drainage outlet in the south of Cindersap Forest. Try interacting from the accessible tile immediately beside the entrance. They lead to the same Sewers and do not require separate keys. Once inside, follow the walkway to meet Krobus. A still-sealed side area in the northwest has its own later quest.