Regardless of the distribution, would it be possible to have a phone that can be simplified to include only the features you want? For example, I would only want a calendar, notes, a calculator, calls and messages. I wouldn’t want a browser, an app store or any other way to easily install them. Is this possible on a Linux mobile distro? Does it require a lot of work, like making a specific distro for it?
Linux feels like a great option for this because it can leave enough freedom to dumb down what you don’t need while also keeping a big level of customization, not everyone needs are the same regarding to dumbphones
Yes, it’s possible. Some things might be challenging, like keeping the system up to date without the user having access to an app repository, but it’s possible e.g. by running a script as a cron job.
Whether it’s a good idea is harder to say. Linux distributions for phones are not especially mature and polished compared to desktop Linux. You might get better advice if you explain why you want to do this and who the intended user is.