I ran some tests on the NES CPU and found the following behavior:

The CPU's status register AND $30 is always 0 (there's no way to actually find out whether these bits are implemented or not, thus it makes no difference). i.e. there is no Break or Reserved bit in the status flag, but those bits are used in the byte which is pushed on the stack during status saves.

PHP         push status OR $30
BRK         push status OR $30, then set I to 1
IRQ, NMI    push status OR $20, then set I to 1
RTI, PLP    pop status unchanged