There is a term called end of support, which means that a device will not get software updates after the support period ends, and as far as I recall, On series is now on end of support, so...no more. And anti viruses are not necessary in Android because it is based on Lonux kernel and Linux is immune to viruses.
My phone is On Max. True, older models do have a shelf life making them redundant to newer updates. But I don't think that's makes the new models with latest OS immune to a security issue
