First, you still get paid for promo codes for titles released before the March 26 announcement. So if you have an audiobook that was released in, say, February 2020 and you just generated 25 codes for it yesterday, you will still be paid when those codes are redeemed.
Next, you don't get paid for codes on titles released after 3/26, but those redemptions still count for ranking purposes. Ergo, even though you aren't paid, you can still get visibility (and perhaps additional sales) from those codes.
With those two things understood, here is how my promo code strategy usually works these days:
1) I use one of the promo code services and upload my codes so that listeners can simply grab a code if they're interested. (It's a lot more efficient than when I used to send them out individually.) Typically, I make the listing "private" at first so that only those with the proper link can see it and redeem codes. I then send said link to my Patreon supporters, letting them know I have a new audiobook and giving them exclusive access to the link for 24 hours.
2) I have an audiobook newsletter (separate and distinct form my "regular" newsletter), and - after my Patreon supporters have had a chance to redeem codes - I inform my audio subscribers and send them the link. I also announce it on my website and Facebook page.
3) At this point, I've made the link public and continue to promote it via social media.
Typically, US codes vanish at a lightning-fast pace; the first batch is usually gobbled up within 30 minutes of #2 above, and the second set will disappear on whatever day I generate/upload them. UK codes are a little slower, but the first batch is usually gone within a day, while the second set may linger, depending on various factors. All in all, however, this has been a successful strategy for me.