I would not "knowingly" fit a piston with a hole like that,
but I have disassembled a perfectly perfoming engine that had 'holed' there.
Since then I only notch to as deep as the first land is high,
and ALWAYS USE A RADIUS IN THAT LOCATION!!
I also set up a simple tool to measure the minimum thickness in other parts of the piston.
The way that notch has been done, it is likely that a piece will break away at some point,
so best would be start with a new set.
If you need more piston to valve clearance there are many other ways.