I believe you are focusing on the wrong word here. The issue is not about "that" but about whether it is "the last time" (as in last week) or "the last time" (as in the final and last time).
"C'était la dernière fois que tu m'as dit que tu m'aimais." -- That was the last time you told me that you loved me. (There won't be a next time.)
-- Chris (not a native speaker).
I don't see any reason to assume "C'était la dernière fois que tu m'as dit que tu m'aimais." means there won't be a next time. How would the sentence have been written to indicate the previous time instead of the final time ?
For me both the English phrases could be read either way and there is no context to indicate whether it is the final time or the previous time.
I don't know if the French sentence is ambiguous or not...so it would help to see two phrases, one for the final and one for the previous....to see how they would both be expressed in French.
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