That M-W sentence means that in the past subjunctive, "were" is correct, even in places where "was" is correct in non-subjunctive. So it strongly implies that "was" is incorrect as a past subjunctive.
I disagree though, or at least I don't think it represents the full modern story of "was" and "were".
It's true that in academic and formal writing "were" is still preferred for the past subjunctive, but when speaking, in almost every context where we normally use "was", both "were" and "was" are correct.
(a) If my brother were here, he would know what to do.
(b) If my brother was here, he would know what to do.
(a) I wish I were at home in bed.
(b) I wish I was at home in bed.
All of these are natural and correct when speaking today, but as little as 20 years ago -- and definitely 40 years ago -- the (b) versions were considered incorrect or uneducated.
The only context where "was" still sounds distinctly incorrect, or at least uneducated, is in the phrase, "If I were you, ..." for introducing advice. That phrase is fixed, so "*If I was you" sounds wrong.
Indulgent tangent: The first indication I had of this shift was in 2004, when Gwen Stefani covered the 1964 song, "If I were a Rich Man". Of course, she changed the word "man" to "girl", but she also changed "were" to "was": "If I was a rich girl". My first reaction was that she'd purposefully introduced bad grammar, but it didn't actually sound wrong, just new, which was almost certainly her intent.