You can use au 📲 to backup you installation files in Termux, i.e. debs. If you find your system unstable after updating, you can try to use dpkg --purge pkg_name and then dpkg --install pkg_name from /sdcard/debs to downgrade packages to a previous version of a package in the hope of regaining system stability in Termux.
You should use au from your $PATH. To see your $PATH, type $PATH +enter/return. Then use mv au destination_directory to move au to your $PATH. au +enter/return will update and backup your Termux in three keystrokes!
Every time you want to apt update && apt upgrade or pkg up, you shall also be backing up your former installation debs to /sdcard/debs by using au. Ideally, you should change /sdcard/debs to an external mirco SD card destination in lines 8 and 10 of au to save space on device.
If you find your system unstable, you can use dpkg --purge pkg_name and then dpkg --install pkg_name from /sdcard/debs to downgrade packages to a previous version of a package in the hope of regaining package stability.
Important: Please use apt -o APT::Keep-Downloaded-Packages="true" install package_name(s) to populate your backup directory in conjunction with au to ensure that you can enjoy a full backup of *.deb files on device.