In order for apps to be able to be updated automatically with the unattended update API introduced in Android 12, the apps being updated must have a specific targetSDK or move. In this case, Termux, doesn't.
That targetSdk level is raised every year, if I recall correctly.
F-Droid is just letting you know that due to this app not meeting that requirement, they can't update it automatically, and you'll have to do it manually instead.
On a personal note, I'd highly recommend not using apps that target lower versions of Android like that. F-Droid doesn't remove or disallow apps with low targetSdk requirements, which is why you're finding a lot of apps like that there. New targetSDK levels can bring significant privacy and security improvements, which is why raising that level for apps is important.