Since I started playing again, anytime I've had the opportunity to "stealth out" and try and perform a quick out of combat revive or "rez," its been impossible. Before, countless times, I've been able to easily revive group mates. Multiple at a time. I recall an instance in a 16m SM EV, had half the group down from poor platforming, and I must have gotten five or six players up before finally being forced back into combat.

Now, on a 16m SM op, I stealth out, start the reviving process, and within a 1, 2 seconds, I'm back in combat. I will also mention I didn't have any buffs/debuffs on any other players/NPCs such as HoTs or DoTs, and I also predicted the mechanics to time my attempts to long before any new enemy NPCs spawned.

And I know it doesn't really matter much anymore because of the Story Mode nerf or buff, depending on how you look at it, allowing combat revives to be used insanely more frequently, but it was unique and cool for stealth classes to be able to get someone up even after the battle rez was used.

So, was there a change in mechanics in Operations and Flashpoints that reduces the capacity to stealth out and revive downed players?

Stealth revives are more complicated than before but still possible.

Most players try to rez by right clicking on the dead player. That doesn't work and you get back in combat. For a successful rez you need to use the ability Revive Player in your ability bar.

other things to consider when reviving. You will be pulled into combat, when you take dmg, get healed by a roaming healing ability (especially the sorc/sage one), new adds spawn or for chain revives most important someone else who is out of combat enters it (doesn't matter if stealthed out or after being revived).

to summarize, stealth rez is funky but still possible.