This has been in the act since the early 1990s, I believe. The current provision reads:
Any of the following persons may vote in accordance with Part 11:
11(d) a person who has been absent from Canada for less than five consecutive years and who intends to return to Canada as a resident.
The question is the intention, not whether the person actually returns. I think for intention it is up to the individual to declare whether they have the intention. A person who has left Canada, has established a domicile somewhere else, and has severed all ties with Canada would presumably not have the intention to return to Canada. So I think it's a question of fact in individual cases, but it would be up to the individual, in most cases, to assert their intention.