Re: "The o serialization format has been removed. As it is never produced by PHP, this may only break unserialization of manually crafted strings."
This little-o serialisation format was used by PHP3 but was never generated by PH PHP4 or above. The deserialization code still recognised it, though, for reasons of backwards-compatibility with PHP3.
However, based on a bit of investigation, it looks like this code has been broken for about 15 years, so although this is listed as a deprecation, in practice it wasn't.
See this Stack Overflow question for a really great answer, with a lot more detail about this: https://stackoverflow.com/questions/65289729/what-was-phps-o-serialization-format-for