Only plural agreement ("The deaths of these people were...") seems possible to me. The most common examples of "notional agreement" are with "collective nouns" like group or crowd that are morphologically singular, but that semantically refer to multiple individuals. Plural noun phrases rarely take singular agreement in English, and I think that when they do, it's usually a matter of a particular word developing a special usage pattern, rather than an example of any more generally productive process of notional agreement. (For example, news is morphologically plural, but takes singular verb agreement: I think this can be seen as the result of a historical move of the noun news from the lexical category of plural nouns to the lexical category of singular nouns.)
The presence of the prepositional phrase "of these people" between the deaths and the verb may make it more difficult to judge the acceptability of was vs were, but I think it's clear that was would be bad in a sentence like "These deaths were/*was to have far-reaching effects for Rome" or "Their deaths were/*was to have far-reaching effects for Rome."
The "singularity" of the event doesn't change based on whether you refer to it as "the deaths of these people", "their deaths" or "these deaths", so I think the examples in the preceding paragraph show that the mere fact that a plural noun phrase can be understood as referring to a single event is not sufficient to warrant the use of a singular verb.
As Robin Betts pointed out in a comment, the singular noun death would be used with the singular verb was.