I think the tag is worth keeping and the case for using it here is reasonable.

@@ -11,0 +12 @@
+<misc>&yoji;</misc>

The Kanji Haitani list is no longer an issue since we removed all the tags (over 1000) which were unsupported by other sources:
https://github.com/JMdictProject/JMdictIssues/issues/72

This entry, 波及効果, is also in a yoji dictionary published by shogakukan:
https://kotobank.jp/word/波及効果-2236979

The gist of the linked wikipedia article seems to be that even Japanese editors have a difficult time categorizing and selecting four character compounds to include in yoji dictionaries. It says the editors of 『四字熟語ひとくち話』 (Iwanami, 2007) argued that it's ultimately a matter of opinion and interest.

In the case of 波及効果, the consensus among dictionary publishers seems to be that it is an idiomatic yoji. I don't know how else we could be consistent with the usage of this tag other than by appealing to these references.

I don't think there's any good reasons to go with them. I think 波及効果 and the 40 or so other XX効果 entries in Jmdict can only be said to be yojijukugo in the very broadest sense - that they're compounds consisting of 4 characters. The original idea back in 2014 was to have a tag for words that are obviously idiomatic or proverb-like expressions, like 弱肉強食. I actually suggested to make the tag [iyoji] for "idiomatic yojijukugo" to make it painstakingly clear exactly what we mean by the tag, but Jim and Rene didn't think it would be necessary. 

(I've long been unhappy with the Kanji Haitani list bulk update and even suggested in 2018 we scrap the yoji tag altogether because the term is just too ambiguous - see https://ja.wikipedia.org/wiki/四字熟語#四字熟語の範囲 - and we're being very inconsistent with how we're applying it)

This is in all four yoji dictionaries that I usually check. Is there a good reason to go against them?

https://yoji.jitenon.jp/yojid/1943.html
https://dictionary.goo.ne.jp/word/波及効果/
https://yoji-jukugo.com/波及効果/