Q:

Is でござる used in Modern Society?

If one speaks Japanese to a, say, stranger in Modern times, is 「でござる」still acceptable?
If somebody says 「でござる」, what would be the people's general reaction?

A:

When my American husband tried using でござる in speech among college-age Japanese friends for fun after reading the Rurouni Kenshin manga, back when he was learning Japanese in the early 2000s, they were baffled and told him to stop. You could toss it out once or twice as a joke, but no: though you could be grammatically-correct in using it, it's not considered acceptable/welcome speech to consistently use in general contexts. If you persisted in it (my husband chose not to), you would likely be viewed as eccentric and/or 空気読めない. It's not "really off-putting" to the level of making people cringe or feel offended, but Japanese culture doesn't generally welcome individuals going against the norm and standing out, so you'd be indicating that either you don't give a crap about that value, or that you don't understand it, or that you're odd.
See also Boaz Yaniv's answer here which explains that

the stereotypical Samurai speech in Jidaigeki is actually based on the Edo dialect of late Edo period. Many of the mannerisms you'd find in this speech do not specifically represent Samurai, but rather a typical resident of Edo in that particular time.

A:

Yes (and no). People frequently use でございます in very polite speech, and nobody would find it strange to hear it used in such a context. Given that it's used frequently in polite situations, though, it will usually take the polite ます form. That is to say I think that it would be much less common to hear someone use でござる in its plain form. でござる is typical speech in fiction for samurai, for example, so if using plain form someone might remark that you sound like a samurai.

