You’re likely thinking KaiOS. They are still contributing what is required under MPL-2.0 but the rest is proprietary. KaiOS 3.x finally got off of a browser from 2016 as the base, but very few have upgraded their apps to be compatible (the tweaks were minor) & others have used it as a reminder that they were still ‘supporting’ a platform like whoever is maintaining or using that WhatsApp thing for chat.
There’s also Capyloon built from B2G, but it’s still early on & is targeting touch phones, instead of feature phones.
It would be nice to see it around IMO since it’d just be another enhancement to progressive web applications & JavaScript is a better target than Java or Swift.