How to keep a specific bit set all the time for bit masking I2C operations?
'static' just avoids reinitialization and is used for local vars in timed interrupts for example.

volatile is another keyword you should not be using without thinking about it and does not appear to be useful for your case. See https://stackoverflow.com/questions/4592762/difference-between-const-const-volatile for more information

The appropiate keyword would be const, which tells the compiler this var mustn't be changed.

But I suspect there is another problem with your code, which will make the compiler throw an error as soon as you use const in the declaration.
I assume you are not using the variable correctly. You are meant to use a bitwise OR | in order to set the required bit along your actual data.

So do it like this:

data_byte |= bitmasking;
write8(data_byte);