To be honest I see robots.txt as a failed experiment since it relies on trust rather than security or thoughtful design.
For example I've got a link to do delegated login like /login-with/github. When people click it an oauth flow will start. But it is useless for robots to follow so I disallow it in robots.txt. If they still follow nothing breaks and it's not a security issue but if I can avoid starting unnecessary oauth logins it's an additional benefit.
However I'm not claiming security is the only reason people use (misuse?) robots.txt. For example in your case you could mitigate your need for a robots.txt with a nofollow attribute[1]. Sure bad bots could still crawl your site and find the authentication URL without probing robots.txt so the security implications there is pretty much non-existent. But you've already got a thoughtful design (the other point I raised) that mitigates the need for robots.txt anyway so adding something like "nofollow" maybe enough to remove the robots.txt requirement altogether.
According to your logic, newspapers are a "failed experiment because they rely on trust rather than security or thoughtful design". I published an article with my treasure map and told people not to go there, but they stole it.
I said following proper security and design practices renders obsolete all the edge cases that people might use robots.txt. I'm saying if you design your site properly then you shouldn't really need a robots.txt. That applies for all examples that HN commentators have raised in terms of their robots.txt usage thus far.
I would rewrite my OP to make my point clearer but sadly I no longer have the option to edit it.
But how? For example, if you don't want a page to be indexed by Google, you add this information to robots.txt. Nofollow doesn't work for every case, because any external website can link to it, and Google will discover it.
<meta name="robots" content="noindex">
Interestingly in that article, there is the following disclaimer about not using robots.txt for your example:
"Important! For the noindex meta tag to be effective, the page must not be blocked by a robots.txt file. If the page is blocked by a robots.txt file, the crawler will never see the noindex tag, and the page can still appear in search results, for example if other pages link to it."
I must admit even I hadn't realised that could happen and I was critical of the use robots.txt to begin with.
Nofollow is a good suggestion of you control links to the resource, robots of you don't.
[0]: https://developers.google.com/webmasters/control-crawl-index...
Using robots.txt to secure your server from bots is the equivalent of attempting to secure your house from robbery by planting a sign that says "please,don't rob my house". Surprisingly it may works from time to time, by if you're into attempting security by wishful thinking maybe don't be too surprised when it fails about as much as security by chance.
If you need to add security (logins) to protect content you don't need to protect you inconvenience users.
Your point about sitemaps helps illustrate that point of mine because having a decent sitemap mitigates the need for Allow lines in robots.txt. It's another feature of the web where robots.txt isn't well equipped to handle and thus there have been other, better, tools built to highlight pages of interest to search engines.
robots.txt was proposed after a badly behaved bot DoSed a web server 20+ years ago, those were different times. With the robots.txt standed now those who wants to play nice can do so without asking anything, for the badly behaved ones it's still up to the admin to put forward the appropriate measures.
I do get what you're saying but if you have to implement "appropriate measures" anyway then the robots.txt file becomes completely redundant.
It should be non-negotiable if you don't want your personal contents indexed by scrapers and archivers, and it should be enforced by design. It's a broken system.