It doesn't apply to all the words, just some. Here are examples of words from that are marked with that symbol
- 悪い (with reading にくい): 大辞泉; 大辞林
- 面倒くさい (with reading めんどくさい): 大辞泉; 大辞林
- 皆 (with reading みんな): 大辞泉; 大辞林
- 判る・解る (with reading わかる): 大辞泉; 大辞林
The thing is even for the same word, 大辞林 and 大辞泉 places the ▽ symbol at different positions.
For your convenience, here are 2 cropped screenshots to illustrate what I mean:
Apparently 明鏡国語辞典 uses this symbol too. Here's a screenshot for 解る・判る from an EPWING viewer.
As far as I know, 広辞苑 doesn't use this symbol. See below
As I'm typing this, I noticed a pattern: the symbol seems to indicate that the reading is either non-standard or not-so-common for that particular kanji. Several kanji references seem to support this interpretation.
Take 悪 for example, kakijun mentions "にく(む)[常用外]", and 漢字辞典ネット labels "にく(む)" with 準1 (which I assume is the second highest level in 漢検?). Same for 判る and 解る, whereas みんな and ど aren't listed as reading for 皆 and 倒 (perhaps those are more colloquial?).
It's just that 大辞泉 and 大辞林 uses different convention: for 大辞泉 it's the kanji to the right of ▽, for 大辞林 it's the kanji to the left of ▽.
Am I going in the right direction with this interpretation?