[meta] Remove other uses of “illegal” from core

Problem/Motivation

This issue is to coordinate all the steps in order to remove all other uses of "illegal" that were not addressed on Re-word "An illegal choice has been detected" message to remove legality suggestion 📌 Re-word "An illegal choice has been detected" message to remove legality suggestion Fixed .
Currently there are 40+ lines of code using the word "illegal", which includes comments, function names, schemas, constraints.

Proposed resolution

List uses of "illegal" with:
grep -ri "illegal" * | grep -v "node_modules" | grep -v "vendor"

Add child issues to:

• Change "illegal" in other UI strings and exception messages and related documentation.
• Change "illegal" in documentation.
• Possibly deprecate API methods etc. that use "illegal" in favor of better-named replacements.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet