"I didn't buy that" without 了 Is 了 ever/sometimes/never omitted for past subjunctive?  What if not subjunctive; i.e., positively something did happen.

wohok.com gives for 没 two example sentences

我没买这个
我没有买这个


and translates both as "I didn't buy this."  (Could argue this is not subjunctive, but I am familiar with languages where a negative is always subjunctive.

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Aside …

What is the difference between 没有+V. and 没+V.? answers that there is no practical difference between 没有 and 没, but a comment suggests subtle differences in meaning or connotation.  (Have to translate a lot of Chinese to figure out what it is saying!)
了 never appears in past negation. 

By definition, one of 了 grammatical functions is to indicate completed action, and not past tense. Hence, if the action did not happen at all (没), it has also never been completed. 


  他还没来 - He still didn't come (and not他还没来了）


A side note to that is that 了 would appear in present negation. 


  他不来了 - He is not coming.