Answer by Nikolay Ershov for How should I understand the phrase "этонесутьважно"?

It isn't. Like @turn_ said, суть an old and plural present-tense form of the verb быть, which strictly speaking has no business being used in the singular.

It doesn't work. But in your authentically old postscripted example, суть correctly agrees with the plural они.

Gaps in the understanding of one's own older language happen all the time. As do occasions when people consciously want to sound archaic. This results in "broken" archaic language. In English, you get ye olde, which isn't a form that ever actually existed; it came from a relatively modern misreading of þe olde, þ being just a fancy old single-letter th. Or, to take a closer example, lots of people would say I thinketh to imitate Shakespearean-era English, which is just as broken as "I thinks".

Этонесутьважно can be translated, with a minor adaptation, as It art not important. Basically, the speaker mistakenly thought art (2nd person sg. pres.) was just a way of saying is with some added archaic gravitas.

The intended effect is something like "it matters not" or "it is of no importance"— i.e. "it doesn't matter" said with a manneristic self-importance.