Minecraft – Has Minecraft mob spawning been changed to reduce effectiveness of vanilla mob farms

Technically, placing torches will decrease your overall mob spawns; the game does pick a single random point within the eligible chunks to spawn a monster, and if that point is not suitable, the monster doesn't spawn. Therefore filling other caves with torches will reduce the number of spawn-friendly points, reducing your overall monster count. This does technically help your mob grinder because you don't have mobs lurking in neighboring caves counting against the mob cap (and ultimately taking it all up, preventing anything from spawning in your grinder).

The best solution would probably be to use a vast water network to drag all the mobs from all nearby caves into your grinder, so you maximize spawn area, but bring any mobs from all the viable spawn points into a single area.

There is a safe radius around the player where creatures can not spawn. Normally (depending on how big of a 'trap' you have built) the player's radius includes a dungeon spawning trap. The Dungeon Spawner is unique in that it ignores this normal requirement for spawning, which is why they keep appearing in front of you.

So, if your no-spawn radius includes the dungeon then you are ok. Also of note is that dungeon spawners will spawn creatures in mid-air and above water, normal creatures can not do this because of the spawning rules. Again, depending on the design of your trap it should be inherently safe.

However if wherever you are standing can be reached by just walking or 1 block jumps or multi-block drops, be aware that something can spawn outside of your safety radius and eventually find its way to you. I suggest only going AFK if you are completely sealed off :)