I think there are several real issues here.
One is the status of the Oracle support, and you are correct that a text file could solve that. Assuming people bother to update it.
I think a bigger problem (for the Oracle minority, anyway) is that Oracle support is seen as dispensable by a lot of people. These are the folks not bothering to make any effort at all to keep Oracle queries in sync with Postgres ones, even where it is easy to do so. I doubt those folks are going to bother to update the file, so it becomes almost worse than a moot point. If I commit this package I'm working on with a note saying that I have brought the Oracle queries up to date, and the same person or persons continue to work in it only on the PG side, my note quickly becomes a lie and worse than not having one at all.
I don't know the solution, and I don't know if there is a solution, but at least I have (hopefully) made everyone aware that there is a problem here.
