There’s some controversy about that.
If you search on the web, it appears that those verses are not part of the oldest manuscripts but were added some hundred or more years later.
I always thought that Jesus upbraiding the disciples just never fit with the way He treated them the rest of the time. That’s certainly not the way He treated Peter or Thomas after His resurrection.
Acts 28:1-5; Acts 5:16; 10:38; 19:11 would give a clear indication that what Mark 16:17-18 states was happening. The signs to follow believers of v 17-18 are also promised in Mt. 10:1-8; 17:20; Mk. 9:23; 11:22-24; Luke 10:19; John 14:12.
The earliest and original manuscripts were written on papyrus...Papyrus didn't last long and for years and years, those manuscripts were captured by the Catholic church and destroyed...
The oldest Catholic manuscripts extant are written on vellum; animal skins, which would last far longer than parchment manuscripts, which by the way was far, far more costly to produce than papyrus...
The majority texts which are the texts of the Reformation and number into the thousands have this verse...
Pretty much every bible out there has this verse including the Catholic Douay-Rheims version...