65C02 is a CMOS version of 6502 (which was NMOS) chip. Judging from Wikipedia articles on these chips and few of the datasheets I found, it seems like it should work exactly the same way it did on 6502, i.e. held high during normal operation, pulled low to fire an interrupt.
Are you sure it is not connected to anything? I'm not too familiar with Apple ...
I've been meaning to get around to this for ages, sorry. Tonight I stumbled across SAMS COMPUTERFACTS for the Apple IIe which I had downloaded from the Internet Archive.
Here is the logic chart data for the CPU (IC UC4) from page 27. I've rearranged it for clarity.
Pin 6 should be high, as you say. (Note: I haven't verified this data on real hardware yet.)
Are old 65C816s TTL compatible?
If their Vih is 2.0v, they are certainly TTL-compatible.
Was there some kind of change?
All WDC chips seem to be verilog-reimplemented, so when they synthesized them into netlist, they've intentionally dropped TTL-compatibility of IO-pins.
The reason for that might be the need to have chips working in 1.8V..5.0V range (...
The W65C02 data sheet does explain it (p. 9):
3.4 Interrupt Request (IRQB)
The Interrupt Request (IRQB) input signal is used to request that an interrupt sequence be initiated. The program
counter (PC) and Processor Status Register (P) are pushed onto the stack and the IRQB disable (I) flag is set to a "1"
disabling further interrupts before ...
Who made your 65C02? Was it WDC?
If it's WDC, then there are a few pins that should be held high with a 3.3k pull-up resistor. Such as the BE pin which is only on WDC versions. BE (Bus Enable) should be pulled up.
Some example taken from http://wilsonminesco.com/6502primer/MysteryPins.html
VPB (or VP):
With WDC's 65c02, leave it unconnected. With ...