Do you mean a Friday Fast rather than the Friday Abstinence?
When did the Church last observe a regular Friday Fast throughout the liturgical year? Not a trick question; I’ve no idea when (or even if).
I’m not sure making that kind of thing mandatory for that purpose would have the effect you intend it to have. If you want to fast on Fridays in reparation for the horrors of abortion, be my guest I imagine fasting won’t do much good if people only do it out of obligation. It isn’t enough to do things “because the Church says so”.
Aside from spiritual benefits, if the policy is that the requirement would be lifted once abortion becomes illegal, then everyone would be much more inclined to actively campaign against abortion. I don’t mean to trivialize your point, but that would be an interesting side effect.
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