What happens if, for once, there is a system update notification, it is being ignored and nothing done?
My first smartphone being HTC, then switching to Samsung twice, i don't recall getting system update notifications every now & then from the mentioned brands.
Depend on the Android OS, Android 8 and 9 onwards.
Now they will have monthly security update to patch vulnerability.
Older phones on Android 4 - 7 there is no monthly patch.
Sony have been active to push these updates, I do not use
other Android phones so I don't know about Sammy and etc.
You can ignore but the patch does several things like bug fixing,
performance, add features to it and etc.
Especially if you have issues with the phone, once the patch comes
it may or may not fix it.
Why not do it?