Paste my reply in another post here in case some who also get interested in this feature.
Thank you for staring a new feature request. This does not seem to be a very common request. Usually we have the Fingerprint or Face ID for authentication on a mobile device itself, you may check if there is any third party apps and that can apply Fingerprint authentication for certain APPs.
If others can get your phone easily and unlock the phone, you could consider logging out the APP every time after using the app.
Btw, Kasa APP and Tapo APP has newly added Two-factor authentication, this means untrusted device need a verification code to login, and you can see what are the logged-in device and manage the logged in device.
Introducing Two-Step Verification (2FA) for the Kasa App
Introducing Two-Step Verification (2FA) for the Tapo App