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Apple is once again advertising on X after more than a year
(appleinsider.com)
This is a most excellent place for technology news and articles.
Yeah, the most "free" option is probably android custom ROMs (anything based on lineageos, I suppose).
There are alternatives that are slowly building up, such as postmarketos, but it's not production ready.
I think KaiOS was a thing for a while, and it's based on FirefoxOS. But you would have to ditch the confort of a real smartphone and I'm not sure it's still active?
EDIT : KaiOS is indeed maintained BUT it is still based on Android (probably with less higher level components)