"That" as a relative adverb

My tentative conclusion:

  1. Try not to use “that” as a relative adverb;
  2. Use “when, where, why, and how” as relative adverbs to begin restrictive relative clauses;
  3. Or, omit “that” if it is possible.

The third point is not clear. I don’t think all relative adverbs can be omitted.

“I got married in the year that aunt Charlotte died.”
“That was the year when Kennedy got murdered.”
「thatが副詞的付加詞としての役割を果たす場合があり、関係副詞、あるいは、前置詞と関係代名詞からなる前置詞句と等価である。」(安井稔 訳)
–A COMPREHENSIVE DESCRIPTIVE GRAMMAR OF ENGLISH by R. Declerck