It won't free you from the distraction of your other devices though, as you've probably got a fully-online phone within reach.
I've been using an Onyx BOOX Max Lumi as my daily driver for about a year. It's an e-ink Android tablet, principally intended as an e-book reader (which I use it for), though it also serves quite well for podcasts, web browsing, and general terminal work (Termux / SSH).
If I really want to minimise distractions, I'll disable networking. I can read local content (articles, or my Pocket stash), which is already ample distraction. Or play back downloaded podcasts.
It is far more usable outdoors or under bright lighting than any laptop or tablet, which is itself highly useful.
Battery life is quite good when reading ebooks. This involves disabling Bluetooth, WiFi, and if possible, the Backlight, and avoiding screen repaints. When used for podcasts or browsing, CPU drain is fairly substantial, though I can get well over a day's use per charge typically.
There's no social media or any other apps on the device. My one authenticating app is Pocket.
The UI is not entirely as distraction-free as I'd like, but it is far more so than any other device I've owned in decades.
Lacking colour and high-speed animations is of and by itself a major win on distraction-free status. So, interestingly enough, is the ability to take freehand notes on the device. I'd not anticipated doing that much, it's turned out to be a significant use.
It’s not very portable, though. I just picked up a Sears Citation for that, but the 1960s typewriter definition of portable is much closer to “you can take it on a trip and type in your hotel room” than it is to “you can take it on a hike and type under a tree.”
For actually portable typewriters I've really enjoyed my Royal Mercury and Smith-Corona Skyriter, both of which are usable on a lap on a bench (or under a tree).
I realise this is not an option for everyone of course. There's also apps that allow you to time lock your phone with exceptions for important things. Protip: set it up so you can get around it by rebooting the phone, in case you forgot to whitelist something, etc. Smartphones are painfully slow to boot anyway, so it still helps discourage the constant distraction if you can muster the will power to not do the reboot. More effort means you consider it rather than reflexively do it.