Inside the source of the Javascript player (base.js) was a function to transform (update) the value of the video signature (s) parameter. All you had to do was look at base.js and duplicate the string operations used to produce the updated s, whatever they were. No need to use Javascript. yt-dl chose to use Python. Only a minority of videos used this "technological measure". Most videos on YT do not require a continually updated s; the value of s stays the same.

2017: https://tyrrrz.me/blog/reverse-engineering-youtube

2015: https://github.com/bitnol/CipherAPI

2014: https://gist.github.com/kl/9070523

2014: https://api.w3hills.com/ytcipher

Regarding the "bookmark" comment, there is no way to "bookmark" any YT download URL because all YT download URLs (not just ones that have a changing signature) include timestamps; as is typical of download URLs on video sites, they have an expiration. Generating URLs that expire is not done as a means of copyright-related "access control"; the purpose has to do with caching.