Ossification comes from os, ossis: bones in Latin. Turning into bones. Stops being flexible. Common behavior becomes de facto specification. There's stuff that's allowed by the specification but not expected by implementations because things have always worked like this.

It's not related to open source software. The seemingly matching prefix is coincidence :-)

https://en.m.wiktionary.org/wiki/ossification

Pretty much when something in the spec in theory could change, but in practice never does. So software and hardware gets built around the assumption that it never changes.

For example for networking you can have packets sent using TCP or UDP, but actually there could be any number of protocols used. But for decades it was literally only ever those two. Then when QUIC came about, they couldn't implement it at the layer it was meant to be because all the routers and software were not built to accept anything other than TCP or UDP.

There's been a bunch of thought in to how to stop this stuff like making sure anything that can change, regularly does. Or using encryption to hide everything from routers and software that might want to inspect and tamper with it.

It's when existing implementations' inflexibility prevent protocol evolution.

https://en.wikipedia.org/wiki/Protocol_ossification

literally it means that something is slowly turning into stone, like dinosaur bones. protocols and standard libraries suffer from this in figurative sense.

Sclerotization.