AFAIK, you still can play offline with alternative launchers like Prism Launcher (you need an account to do the initial setup and download, but once everything is downloaded, you can add as many offline accounts as you want), and the downloadable server still has a "do not validate the accounts online" option which allows offline users to connect and play.
Not the most approachable option, but you can also setup a mod dev workspace and launch that way. Or download and launch the game directly with a `java -jar ...` comand.