I can't say I have a limit. But I highly doubt I'll go electronic, disc brakes or tubeless.
But I've taken 80's bikes to 11 speed with modern wheels no issues and kept other pure stock and pretty much everything in between.
85 kicked up to 8 speed STI and carbon tubular rims. A mild upgrade.
87 with full Campagnolo 11 speed and Zonda rims
85 ummm.....downgraded with 81 tubular rims.