しろ vs. せよ imperative forms

Question

While browsing Youtube, I came across a comment which said the following:

はじめしゃしょーを思い出した人は返信せよ

My question is about the ending. I understand that せよ is an imperative form of する just like しろ. However, I was wondering what difference, if any, this makes to the meaning of the phrase?

I found an explanation on another forum saying that せよ is more literary-sounding, and is used in exams. However, I wouldn't have thought that a Youtube commenter would use particularly formal language, so I wonder if there's more to it than that.

Apologies if I've made any mistakes in how I asked the question, this is my first time posting!

Answer 1

せよ is the imperative form of the verb す, which is an archaic verb meaning する in modern Japanese. This archaic imperative is still found in modern Japanese, typically in examinations at high school and university. It generally looks highly stiff, blunt and authoritative.

In this case, however, this せよ is used more or less jokingly, to make this message look as if it were a military telegraph in the mid-20th century, or a message from a boss in a spy movie (e.g., "～を抹殺せよ. This message will self-destruct in five seconds."). Plainly saying 返信しろ would have been too rude, but 返信せよ would sound like the listeners were given a critical "mission", which might make them excited.

Answer 2

As you said せよ and しろ are both imperative forms for する. The first one is historically older. Therefore せよ sounds more formal than しろ. As you mentioned せよ is often found in exam questions.

The one who commented probably wanted to sound like "an exam question" for the fun. I don't think there is more to it.