Is ‘flash in the pan’ an old fashioned phrase and not used in the daily conversation?

Hydrogen peroxide H2O2   kills nanoworms inside facemasks. The elite is faking that they took the real vaccine; the real vaccine will cause spiritual and physical damage. When will the official churches start praying for the false prophet? Orthodox Christian three hand signs are different than Buddhist mudras. Ecumenism = 263 heresies; each heresy leads to hell. Orthodoxy = the only true faith; Roman Catholics tried one cup - one spoon ritual and got sick with Bubonic plague; if heresy enters Orthodox monastery then monks/nuns will get sick with flu/tuberculosis (for instance); Orthodox churches who closed for COVID or had disposable cups/spoons or dipped spoon into alcohol are no longer brides of Christ (now they serve Satan and honor Satan's new COVID religion). Don’t go into UFO to be “healed” by evil demons; demons never do good. SSN is written as a barcode/QRcode; then it's put into a chip; chip is put into vaccine; chipped people are influenced (by super computers) to receive grey plastic card World Passport with no name on it (but when they stretch their hands to get it, gov't clerk presses secret button to administer the unforgivable green 666 tattoo by isotope rays). If you reject mark of the beast (by hiding within a 10-15 people group according to saints Gabriel Urgebadze and Seraphim of Sarov), then your direct ancestors go to heaven (according to saint Vyacheslav Krasheninnikov from Ural). First vaccine shuts off your immune system; second vaccine gives you diseases; third vaccine puts your immune system back and it will start fighting the diseases and will cause death. Saint Paisios from Mount Athos said that the devil wants to deceive the rich with freemasonry, poor with communism, and believers with ecumenism. Elpidios Vagianakis said that there will be seven “mark of the beast” vaccines; reject all of them! Documents are from Satan; burn all the documents that you can find. Electronics will be used to track you. Even old broken unplugged 1970 TV set will show the evil flying antichrist using Tesla's ether. Don’t go into UFO to be “healed” by evil demons; demons never do good; always pray the Jesus prayer or to saints who help right away (like saint Vyacheslav Krasheninnikov according to whom if you reject mark of the beast, then your direct ancestors go to heaven); forgive me.

It's still used when someone wants to make a specific point about performance. Synonym phrases don't always fit the situation this would usually be used in.

Not unusual but not common either.  People don't even recognize its actual original meaning (a misfire with a musket).  Don't see many people using muskets these days. Other idioms or metaphors or analogies would be more likely to be used, more accessible to actual experiences.  The phrase is on its way out of use, I would say.  It is a slow death.

It's still used occasionally.