File nomenclature is indifferent to MVC or any Patterns design.
Including ". inc" and ". class" was an old practice that even many older PHP programmers disagreed with the use.
Basically, these letters only helped to identify what kind of file it was. If it had the ". inc" in the nomenclature, meant that it was an inclusion file, ie should not run it directly. Ídem for the types ". class".
There are also the types ". func" and ". cons", for functions and constants respectively and probably there are others, but these are the best known.
Normally these files should be in a private access folder, not accessible by the user. But not everyone could follow this recommendation due to N reasons. Lack of knowledge or resources (the host did not allow).
In general, you will find nomenclatures of the type
file.inc.php
file.class.php
file.func.php
file.cons.php
Some omit the .php
of the end, but this can be a bad idea because it can expose the codes in public if the environment is not well configured.
Old and widely used systems, such as Phpmyadmin, still use this practice.
It is worth noting that it is also not a bad practice. It is merely a matter of opinion/choice of who creates the project. The important thing is that the project is well written, well documented and organized. Nomenclature does not matter as long as you know what you are doing.
About existing other standards and MVC not having to do with web I already knew, my doubt was regarding the use of the inc extension with use of MVC, about the PSR I had already read some things, but I did not find any more complete content explaining the differences of each etc
– Otavio Fagundes