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 (...