See also T187060: reserved Lexeme IDs and T223997: Add reserved IDs to EntitySchema
Other than examples and bootstrap contents, we need to reserve IDs used in builtin objects. The reservation should be forward-compatible as new builtins may be introduced, so we will have some ZIDs reserved but leaved for future use.
I propose to reserve Z1-Z10000.
Note in my proposal "reserved" ZID including both builtin (things hardcoded in the code and can not be removed), standard library and examples (both are uploaded to wiki and may be edited or deleted in its normal way; the only different between them and other ZObjects is you can assume they have a constant ZID).
This is my proposal:
(if more reserved ZID are needed, feel free to multiply the bounds, or use a new range such as Z5001-Z6000)
IDs larger than Z1000 is not intended to be used in Wikilambda itself. They are used for other custom extensions (compare relation of WikibaseLexeme and WikibaseMediaInfo with Wikibase):
See also Abstract Wikipedia/Reserved ZIDs on Meta-Wiki, which references this task.
And Abstract Wikipedia/Updates/2020-11-25.