tl;dr: There is very little, if any, purpose to maintaining the current rewriting and redirecting mechanisms for Amazon links in Stack Exchange posts, since the redirecting mechanism no longer adds SE affiliate referrals to those links. Also, in many cases, users can't follow such links despite the URL in the post being correct, either due to bugs in the mechanisms, or because their network blocks connections to the redirect server.
Since 2009, Stack Exchange has been rewriting links to products being sold on Amazon.com (only the U.S. site, not other international Amazon sites) to add in its own affiliate referral code to the link, so that if someone follows a link someone put in a post, and goes on to purchase the item in question, Stack Exchange would earn a commission of the purchase.
Basically, whenever the Markdown renderer sees an Amazon link, it will rewrite that link in the rendered HTML to instead go to
rads.stackoverflow.com, which is a redirect domain which, for many years, has redirected the user back to the Amazon site, but with Stack Exchange's affiliate referral code in the URL.
However, as of October 4, 2019, Stack Exchange stopped inserting its affiliate code into the redirects generated from
rads.stackoverflow.com: if you follow an Amazon link from Stack Exchange today, it will still have been rewritten to go through that URL, but the redirect that that domain spits out no longer has an affiliate code, so Stack Exchange no longer makes a commission on them. It therefore doesn't have a real purpose anymore.
This rewriting and redirecting mechanism is intended to be transparent: a normal user who doesn't check any URLs would just be directed to the relevant item and make their purchase without knowing about the redirecting that has happened, and (in the past) this would earn SE an affiliate referral fee (without the user being charged any extra). However, this can result in an annoyance to many users, for two main reasons.
First, bugs in the rewriter or the redirect domain can result in links unexpectedly breaking when pasted into Stack Exchange posts or comments. For instance:
There was a bug where links to promotions or item listings (and not specific items) were being incorrectly rewritten and becoming broken. It took several years for it to eventually get fixed, and could crop up again if Amazon comes up with some new URL scheme that mistakenly triggers the rewriter. Plus, previous posts made before the bug was fixed that contain such links will have to be re-edited to fix the links.
There was a (now fixed) bug where links to the international Amazon sites (e.g. Amazon.co.uk, Amazon.in, etc.) were being incorrectly rewritten into links to the U.S. site, breaking them.
In April 2020, the redirect server went down, causing all Amazon links network-wide to be broken.
Finally, as the link rewriter is essentially dependent on Amazon links being formatted a certain way, if Amazon decides to change the format of newer links, it can result in tons more broken links until it's fixed (and posts are rebaked).
As all these bugs require resources to fix (the second one above wasn't fixed until an Amazon employee responded to the bug report), I began to wonder if the affiliate referral fees were enough to offset the costs of maintaining the program, and the response at the time was that the net gain was high enough (far higher than the responding employee's salary every month).
Second, these days, I've been using an ad-blocking tunnel app on my phone that blocks all connections to advertising servers, and it also blocks the server through which Amazon links are directed. This means that I can't follow any Amazon links within Stack Exchange unless I disable that app, which I don't wish to do. This not only affects users who semi-manually block connections to the redirect server: some company/public networks may block connections to known ad servers, which (given my experience) that one may be listed as.
So, given that the link rewriter no longer makes the company money, that Stack Exchange needs to spend time and resources fixing ongoing and potential future bugs that come up with it (making it a net negative), as well as it being annoying to users who use network-level ad blockers, can we get rid of it?
Someone previously commented about one potential use of link rewriting: that if someone attempts to put an Amazon link with their own (or any different) affiliate referral code, that will be stripped out. However, I don't think spending resources to maintain this whole redirect mechanism is worth this one advantage. While it is true that putting in your own affiliate link is considered spam, we don't get as many users doing this with Amazon links (in particular, Amazon doesn't simply allow people to just sign up for the program; one has to have a business or organization in order to do so). Additionally, for the rare case where it does occur, we have community tools such as SmokeDetector that can check for and report affiliate links.