Alternative for .inc files


I was wondering what is the correct alternative of .inc files in Drupal 8? That is to say, if I want to create some functions that should be only be loaded when required, like it was with module_load_include() in D7. Though this function is still there in D8, I hear that this is not the best procedure. Is that so?

I have read somewhere, that using PHP traits is considered a better practice here. However, not sure if useing a trait inside a .module file is a good approach.

Any help?

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