As phones age, the rate at which they receive software updates slows down - this is exactly what happened to several Samsung phones.
The Galaxy J3 Pop/Emerge, Galaxy A7 (2017), Galaxy A5 (2017) and Galaxy A3 (2017) will no longer be receiving any security updates. The quartet had a good run - it went on sale four years ago (January 2017).
There are some more bad news. The Galaxy A8 (2018) was demoted from monthly to quarterly updates while its sibling, the Galaxy A8s (also from 2018), falls to the "other regular security updates" category (those aren't updated on a fixed schedule, only once in a while).
New phones are not guaranteed to start life in the fast lane, that is certainly not the case for the affordable models. The brand new Galaxy A02 and Galaxy M12 are starting off with quarterly updates. And the M12 is so new that Samsung hasn't even announced it yet - that is expected to happen next week.