Hello,

Here is another take on the possibility to exclude headlines from the
table of contents.

Last attempt was rejected because users need to include /unnumbered/
headlines in the TOC. Point taken. However, I don't think it makes sense
to exclude /numbered/ headlines from the TOC. Hence, I propose to
exclude any headline with an UNNUMBERED property set to "notoc".

So basically,

  - only unnumbered headlines can be excluded
  - we do not introduce another property

Here is an example document, with the generated TOC in plain text
export:

--8<---------------cut here---------------start------------->8---
#+options: toc:t

* H1
:PROPERTIES:
:UNNUMBERED: t
:END:

* H2

** H2.1

** H2.2
:PROPERTIES:
:UNNUMBERED: notoc
:END:

*** H221

** H2.3

* H3
:PROPERTIES:
:UNNUMBERED: notoc
:END:


* H4

* H5
:PROPERTIES:
:UNNUMBERED: t
:END:
--8<---------------cut here---------------end--------------->8---

gives

  H1
  1 H2
  .. 1.1 H2.1
  .. 1.2 H2.3
  2 H4
  H5


I attach a possible implementation, with documentation and tests. Note
that this doesn't work with LaTeX export back-end, which still dutifully
ignores any unnumbered headline. Patches welcome to improve this.

WDYT?


Regards,

-- 
Nicolas Goaziou                                                0x80A93738