I have already flagged it to a moderator who asked me to edit the question and ask to reopen it on meta. I ask the seniors here to think about reopening this:
I was searching for how to deal with a slash phrase and had to read through the jungle of the "non-duplicate" at "A/An" preceding a parenthetical statement for a while.
Non-native speaker here. I agree with the questioner, this is not a duplicate, and has not even been answered yet. I had to guess from english.stackexchange.com/a/36326/410036 the following: The a/an that you would read out loud for the first word of the slash phrase is all that you need, since any slash is just a shortcut for "or" + the freedom of saying "a" or "an", that is, the indefinite article of your choice. It took me way too long to find out about this. If this comments is right, the question should be reopened to get this answer, but from a native speaker.
(remark afterwards: not only english.stackexchange.com/a/36326/410036, but also a few more answers below that help to find out about this)
The questioner has asked to un-duplicate/reopen this for four years, to no avail:
NOT A DUPLICATE!
The other question only asks about parenthetical phrases, not slash phrases.
The other question's parenthetical phrase (answer(s) explained that it's not really a parenthetical phrase) is a separate word. Mine is an intra-word parenthetical prefix. That's different!
I see that "A or an before parenthetical phrase equivalent to slash phrase?" is not the strongest point of why it should be reopened, since the answer to this is the same in both "duplicate" and the "non-duplicate": write the first indefinite article how you would read it aloud and leave the rest. While the "A or an before slash phrase?" is easier to understand and search.
One could edit the question and remove the "parenthetical phrase equivalent to slash phrase" and then reopen the question. But then, some input would get lost on this Stack Exchange, since the other question is not about such a "parenthetical phrase equivalent to slash phrase", but about a word on its own that stands in brackets before the noun. Which has the same answer somewhere in the lower voted answers, but you find it only after a picky search.
That is why as a non-native speaker, I guess that the questioner might just be right. And that this question should just be reopened as it is.