Does a macbook SMC reset if you disconnect the battery?
This answer is based on the datasheet from a similar power management controller unit, or PMU (what Apple calls SMC - they're the same thing) as datasheets for those chips are not available.

I am using the datasheet from the TM4C1231C3PM which is approximate to the (Apple SMC) LM4FS1AH5BB  which are both made by Texas Instruments.

There are 6 conditions which will cause a reset:


Power on reset (POR)
Brown out detection 
Software initiated reset
A Watchdog timer reset
MSOC (system clock) failure


The POR will reset the SMC

Wait....but I turn on/off my laptop all the time.

This is the power on state of the chip, not the machine.  As long as power is applied, it will hold the settings which is why disconnecting the the battery resets the SMC.  This is the initial power up of the chip.


  The internal POR is only active on the initial power-up of the
  microcontroller and when the microcontroller wakes from hibernation.


So, to answer the questions...


  Will the SMC reset if you disconnect the battery and press power for 5
  seconds in newer Macbooks?


Yes.  Removing the power and discharging any residual charge will reset the SMC.


  Will the SMC reset if you press the keyboard combination
  Shift-Ctrl-Alt and Power in older Macbooks with user-removable
  batteries?


It's impossible to know without actually testing them as datasheets for older SMC chips are even more difficult to source.  (IMO) it's unlikely since the desktops of that vintage didn't use the keypress and relied solely on removing power, the laptops did as well since they had removable batteries.  It wouldn't hurt to test this.