Hi Ihor,
   Here's the updated patch using :safe, and an additional
patch for the news entry to make it easier to apply the
core change to bugfix if needed. Best!
Tom

> I am also wondering if we should include this into bugfix.

I can vouch for the fact that trying to work around this in
any other way is going to be a massive pain. For example,
it will be hard to use vanilla 29 for various CI types of things
where someone might need to execute a cell but not
want to allow arbitrary codeblocks.