It's effectively dead today, and while it was dominant, Google abused it to get people to view and link to their cached copies of AMP pages, rather than the original site.
It also led to widespread abuse where the AMP version of the page differed in content significantly from the regular page. The same issue existed for WAP.
You may also remember browsing the web using Opera Mini, which wasn't a direct user-agent but used Opera's backend systems as a proxy that stripped, minified and compressed HTML/CSS, and resized and recompressed images. It let you browse the web using massively less mobile data, but raised a lot of privacy and security issues.