According to Apple (https://support.apple.com/en-us/HT212227)
An AirTag, AirPods Pro (2nd generation) charging case, or Find My network accessory that isn’t with its owner for a period of time will emit a sound when it’s moved.
It’s a feature of AirTags that they will emit a sound when separated from the paired owner after a period of time. (You’re correct in your assumption it’s a stalking protection feature).
As it stands however, as of right now, there is no way to stop the beeping and keep the tracking feature active.
The Airtag can either be disabled outright which stops the beeping and stops the tracking capabilities. Or your partner keeps an iOS device nearby that supports the find my network (cheap supported iPod touch for example) and you share that AirTag with their Apple ID.
Apple doesn’t have a middle option where tracking is enabled and it doesn’t make a sound when not with its original owner.
There is an Android app from Apple called Tracker Detect that does allow an Android user to locate a nearby AirTag by purposely making it beep to locate it, but there’s no further capabilities beyond that at this moment.
Software aside. You could just tape the AirTags so that the sound is greatly reduced that way they’re less bothersome by putting pressure on the metal backside of them since that’s where most of the sound comes from but that’s a bandaid solution.
