Quote from: npuath on Today at 10:46:10 AM

Thanks for the tip link. Right now I'm mostly concerned with mined blocks (but no longer part of the longest chain).

Quote from: DaveF on Today at 03:51:36 AM

In the end, transactions do not just disappear.

I wish we could be absolutely certain of this, then we wouldn't ever require more than 1 confirmation, or even PoW at all

For small amounts, 1 confirmation seems to be commonly accepted.

I realise that it's uncommon, thanks to your link and some more research I can confirm that the number of "extinct" blocks dropped from several per year to only 2 in 2019, and actually none since then.

Unfortunately I can't build my system based on hope alone. How do existing payment systems handle the situation when a transaction in a previously confirmed block actually disappears (perhaps because of a malicious actor trying to double spend)?

Once again it depends on the payment system and what you are doing.
Some examples :

With coinex after 1 conformation I can trade on their platform with the funds, but I can't withdraw anything I traded for or the original coin until 6 confirms.

With Bitrefill if I DO NOT use RBF and a high enough fee it's either 1 confirm or at times 0 if it's for a small enough card since I have been using their system for a while.

For larger amounts it's 1 confirm with no RBF and high enough fee. BUT, we are talking about $10 to $50 gift cards, not sure if that is going to change for a $1000 amazon card for a new user.

shoppy.gg is usually 2 or 3 confirms.

If you are going to be gaming the network to doublespend the time and effort involved are going to be a lot more then a few dollars.

-Dave