Something to keep in mind is that there is a double taxation treaty between South Africa and Canada. The conditions in this treaty take precedence over either country's domestic laws, even if South Africa passes this law. According to Article 4 of the treaty and to Canada's definition of a tax resident you can be considered a resident of Canada for tax purposes if you have a permanent home there and not in SA (regardless of how long). This means that you only have to pay tax in Canada and not in South Africa, regardless of SA's own laws (some exceptions e.g. for different types of tax etc.). As far as I can tell, if you still have a permanent home in SA as well then this might be different, but if you don't then you can be considered a resident of Canada only, for the purposes of the treaty. This means that this law won't affect people who have a home in Canada and not in SA ("he shall be deemed to be a resident only of the State in which he has a permanent home available to him"). To change this they would have to change the actual treaty which they are not doing at this stage as far as I know. All of this is separate from South African citizenship i.e. you can be a citizen of South Africa but not a resident and therefore exempt from paying tax to SA if you are already paying it to Canada, according to the treaty. Personally I don't think it makes sense to give up your citizenship because of this law as you probably won't be affected by it either way. I would consider giving it up only if they decide to change the treaty in the future so that you have to pay tax in SA and Canada.
I don’t think 99% of people who’ve emigrated need to worry about this. From what I understand it would only apply when you are an SA tax resident as @pretor mentioned, meaning you would need to be in SA for more than 180 days or something. I think SARS is annoyed because they have no clue how many people actually emigrated and the ones who did probably don’t do their annual returns, they just ignore it and it shows as outstanding on SARS side.
Is the key to this not this sentence from the article: "It states that South African tax residents abroad will be required to pay tax to South Africa of up to 45% of their foreign employment income, where it exceeds the R1 million threshold."?
If you do not meet the definition of a South African Tax Resident, would this still apply to you? I believe not. See here for SARS definition of a "Tax Resident".
THIS explains it further.