I read that EU citizens who have resided in the UK for 5 years and haven't been issued a PR can either apply for settled or pre-settled status, depending on certain conditions, but what about those EU nationals willing to settle in the UK after the formal date of Brexit, or even further on, after the transition period? Will they be required a visa to enter?
During the transition period, free movement continues to be in effect in the UK. EU citizens and nationals of Switzerland and EEA countries can settle in the UK as before. After the end of the transition period, whose who arrived before the end of the transition period can remain under the settlement agreement scheme. For those who arrive after the end of the transition period, it is not yet known what the requirements will be, because that is to be the subject of negotiations during the transition period.
There is perhaps a small chance that the UK will change its policy and accept continued participation in the free movement system, but this is extremely unlikely. In the rather more likely case that free movement ends, then people who arrive after the end of the transition period will probably need to meet the same requirements as anyone else arriving from a visa exempt country: for short visits for certain purposes, they won't need a visa, and for other visits or to establish residency in the UK, they will need a visa.