So you want bunkers that are un-discoverable to both players and zombies then? Again it sounds like you want bunker simulator while in the bunker and zombie/PVP game when outside the bunker. I'm not saying this is a bad thing but it seems like it's the type of game you want to play.
Speaking as a former bunker-lover, that's about right in my case.
My playstyle (and that of the people I play local multiplayer with) has always been...
- Go out during the day to loot/scavenge, but have a safe base to come back to at night to sort the day's pickings, cook, and craft.
- Use melee 90% of the time, with bows 9% and firearms only in emergencies (the other 1%).
- Mining is tedious and anti-fun, so none of us do it, as is building big complex structures.
In A15-A16 this would work fine with a bunker base. We'd spend the night in the base where the lack of zombie digging would mean we (and more importantly our stuff) were safe. On horde nights we'd have a separate platform over a bed of spikes - simple enough that it wouldn't require lots of boring mining for the materials, and the fact that the zombies would (if they got through the spikes) run round in circles underneath us rather than bashing our supports meant that it wouldn't require lots of maintenance and repair either. Similarly, using melee and bows meant we might not have killed all the horde-night zombies by the end of the night but we never needed to spend ages mining for materials to make gunpowder or the like.
In A17, the changes to zombie AI changed all that. I've not dug
anything underground since A17 dropped. After a bit of a shaky start, I managed to work out good ways to keep zombies away and now I always use an existing POI - sealing up the ground floor and putting a zombie-proof entrance on the upper floor. The new AI makes above-ground bases just as safe as bunkers used to be, and upgrading the base from wood to brick to concrete is pretty much just for the sake of paranoia these days. Wooden bases are (assuming you have a separate horde-night structure of course) perfectly safe in A17-A18. They're just as safe as bunker bases used to be.
Horde-night structures were a bit more of a fiddly problem to "solve", but having established that we could make them safe we simply turned horde-night off. It's just not worth the time that you need to spend doing boring repair and resource gathering work. If we want some excitement we'll go do a quest at night instead.
Anyway, I've rambled a bit there - but my point is that as someone who enjoyed having bunkers that were zombie-proof; yes, the ability to have that is no longer there. But I don't miss it, because it's so easy to build a zombie-proof base above ground these days.