contact form customization page is out of date
https://github.com/plone/documentation/blob/5.1/develop/plone/functionality/contactform.rst is no longer correct. It is not possible to customize the contact form that way because Plone 5 no longer uses PTs and CPTs.
It is also currently not possible to customize the contact form TTW, EXCEPT maybe by
- injection of JavaScript
- CSS trickery
- Diazo theme tomfoolery
Oh it's as easy with Diazo tomfoolery as I'd hoped!
<!-- change wording of contact form and thank you -->
<rules if-path="/contact-info">
<replace css:content="#content > div > div.documentDescription">
<div class="documentDescription">Fill in this form to book Joey!</div>
</replace>
<replace css:content="#content > div > p">
<p class="documentDescription">Thank you for contacting me – I will get back to you as soon as I can!</p>
</replace>
<replace css:content="#global_statusmessage > div.portalMessage.info">
<div class="portalMessage info">
<strong>Info</strong>
An email has now been sent to Joey
</div>
</replace>
</rules>
Changing the instructions at the top of the form:

Changing the status message and the thank you message:

@tkimnguyen is this issue relevant for Plone 6? If not, then let's close it. If it is, then let's change the label 99 tag: Plone 5.x to 99 tag: Plone 6.x so it gets some attention from whoever might use it and create a PR for it.
Closing as no follow up from @tkimnguyen. Please reopen if you have details.