While not actually listed in any of the hotfix notes from 18.1.0 and up, enemies within 5 meters of an enemy that you stealth kill with a melee weapon (whether you do a finisher or simply kill them with a melee swipe) will become automatically alerted. This alerted state is dubbed the "Cautious" state on the Enemy Behavior page as they often will not run to consoles, and just simply aim their weapon and walk around suspiciously.
After about 20 seconds, they will return to an unalerted state however. The biggest issue of this besides them not bothering to list it in the hotfix notes (they also seemed to have semi-fixed the bug where having Zenurik focus on will prevent energy gain from syndicate blasts that give you energy, although sometimes it still dosn't work, without bothering to mention it in the hotfix notes either) this causes pretty serious problems with players trying to rack up stealth kill affinity, which is probably why they even bothered doing this in the first place.
Another issue is how stupid the AI in warframe can be, as sometimes enemies randomly will spawn alerted, so all the enemies in a room will be unaltered while one enemy runs toward you and knows where you are for no reason. This is amplified by the new stealth nerf, as there seems to be a chance that when one enemy is alerted, other enemies will also become randomly alerted, and often those enemies will be standing up as if they aren't and you kill them and lose your stealth multiplier.
Another note: Before this change enemies would still become alerted if they see their ally being killed with a stealth finisher, but this has also been extended so enemies will become alerted if they see an ally die within their field of view, regardless of the cause of their death. (killing them with melee or blowing them up with a silenced tonkor). While stealth multipliers is still possible even with rifles, killing groups may be more difficult as some enemies will become alerted for a while.
For all intensive purposes, this change is not a true nerf to the way players gain focus, but it will take longer to do so and you will get less reputation/focus if you do it without paying attention to other enemies :)