Unpkg serves whatever is published to NPM, and if it's a library intended for the browser, that often includes minified versions ready for use in script tags, for example, https://unpkg.com/mithril@2.2.2/mithril.min.js. Sometimes the default export is CJS (which has require() calls), in which case, you can usually use the browse url that I mentioned to see if there's another export you can use.
https://esm.sh/ is definitely a good option too if you're OK with modules.
Yeah but that's rarely the case, you can't offer that as a generic "solution" to the problem at hand. It's just Mithril's choice to both publish to npm and to officially link to unpkg. Packages can make different choices, like publishing to GitHub releases or just to include the file in the GitHub repo.
The reality is that only frameworks and very popular browser-specific packages do that.
https://esm.sh/ is definitely a good option too if you're OK with modules.