In Android 14, Google introduced new limitations on the installation of applications targeting old versions of Android.

What’s changing
From Android 14 it is no longer possible to install any application that targets an API level below 23 - Android 6.0. Attempting to do so will trigger a security exception //

It's certainly better to ensure apps are targeting the latest API level where possible, but as long as applications target an API level of 23 (Android 6.0) or higher for Android 14, and 24 (Android 7.0) or higher in Android 15, apps will continue to be able to install without issue. //

Yes, on an Android 14 device you may connect to ADB and sideload an application with: