Is Pharo considered an esoteric programming language? Are esoteric languages being featured during the challenge?
No, Pharo is not esoteric. It is a descendant of Smalltalk, one of the early and very influential object-oriented languages. I personally don’t like object-oriented programming and languages in the “pure object-oriented” style of Pharo are not very widely used today. But that doesn’t make it esoteric.
The featured languages can be found here, where a purple heart means a language will be featured for that exercise.
No, it’s a real language. It builds on Smalltalk, which is one of the most important and influencial languages every created 
)According to Wikipedia,
An esoteric programming language (sometimes shortened to esolang ) is a programming language designed to test the boundaries of computer programming language design, as a proof of concept, as software art, as a hacking interface to another language (particularly functional programming or procedural programming languages), or as a joke. The use of the word esoteric distinguishes them from languages that working developers use to write software.
Pharo isn’t an esoteric language because developers have used it to write software. See GitHub - pharo-open-documentation/awesome-pharo: A collection of awesome Pharo libraries, tools, frameworks and software. for a non-exhaustive list.