Update the documentation regarding deprecated features planned to be removed and new keywords introduced in v0.9

There are features marked as deprecated and planned to be removed, many of them in the next breaking release v.0.9, which are listed in #15795 .
We also need to document the promotion of many identifiers to keywords, as listed in #14770.

While some of the necessary modifications are already done during the PR to effectively deprecate the features and add the new keywords, the documentation and included examples should be revised and adjusted accordingly to reflect the changes.
A new breaking changes page for v.0.9 should be added to track the removed features and introduced keywords, as well as any other syntactic or semantic change.

Start by reviewing issues #15795 and #14770 to collect the deprecated features and identifiers promoted to keywords. Search the documentation and included examples for affected syntax, then identify the appropriate location for a new v0.9 breaking-changes page. Done means the documentation and examples reflect the changes and the page records removals, new keywords, and other listed breaking changes.

Written by the indexing model from the issue text.