In several fields of science, the equation E = mc2 is not of commun use in practice, although it can be mentioned as being of general interest.
Thermodynamics is one of them and this peculiarity is associated with two others, constituting an interesting trilogy that can be summarized as follows:
1st Peculiarity: It is not inadvertently that the equation E = mc2 is generally absent from thermodynamics textbooks, but because it is classically admitted that this equation seems unnecessary in this specialty and can therefore be deliberately omitted.
2nd Peculiarity: The main reason for this conclusion is that the theoretical predictions obtained by the conventional approach of thermodynamics are generally in good agreement with the results experimentally observed. As a logical conclusion, the usual equations of thermodynamics are considered adequate.
3rd Peculiarity: Despite the relevance of this argument, it is a matter of fact that understanding thermodynamics is known to be difficult and this situation is mentioned in the preface of many books dealing with the subject. This is sometimes followed by complementary information saying that after some practice this impression disappears.