It is not meant that way at all. I mean "any developer's personal choice at time of impl" because the standard allows such versatility. Since in this case you posed it to me its "you as a dev's personal choice". There are 3 options:
1. It is not standardized, everyone is free to choose.
2. There's an informal convention, turning de facto standard.
3. It is formalized, part of the standard.
When saying "protocol decay" I always mean as the IETF doc on it defines it: https://www.rfc-editor.org/rfc/rfc9413.html#name-protocol-decay
On "evolution". Evolution to where? What are the common goals, the shared vision? When is something evolution vs. devolution if people are hemmed into a de facto standard full of app-specific choices made in the past?
"I can't have more profile fields". Why? Does Linked Data restrict you, or does some app not handle more than N fields? The latter is protocol decay.
I've been tooting about Emergence. Many emergent systems in Nature have no interesting outcome. What fedi is emerging?