Nominative Use and Owner's Rights
The WhatsApp brand guidelines page seems to claim rights and impose restrictions not authorized by US trademark law, or the law of any country of which I am aware.
Use the WhatsApp name and logos found on our WhatsApp Brand Guidelines website only, and not those found anywhere else.
If a person is using one of the whatsapp marks to refer to that company or its programs or services in a critical comment, that is valid nominative use and the company may not control what form of the mark is used, provided that no false impression of approval or sponsorship is given.
Do not use other trademarks, names, domain names, logos, or other content that could be confused with WhatsApp.
The company has no right to prohibit such use when a person us making proper nominative use of a mark.
When you're talking about WhatsApp, always capitalize the letters "W" and "A," and never modify or abbreviate the word "WhatsApp."
Again, If this is proper nominative use, the company has no right to control the Form of the use, as long as no false impression of sponsorship or approval is given;
Wikipedia for example, forbids its editors to comply with such stylistic requirements, stating:
From among those, choose the style that most closely resembles standard English – regardless of the preference of the trademark owner.
WhatsApp doesn't permit or license its trademarks, logos or other intellectual property for use on merchandise like toys, apparel, accessories or packaging without explicit prior written approval.
Since the marks are not registered for or used on such goods they probably get little or no protection in such a case, unless they are classified as "famous" marks.
Avoid using the WhatsApp name or logos, or a combination of these, as a verb.
As long as this is not part of an attempt to improperly use their marks to create confusion, trade on their goodwill, or otherwise falsely imply approval or sponsorship, the company has no right to control the grammar of comments about it products.
Other restrictions on that page might not stand up to legal challenge either. However, such a challenge might be costly, even if successful.
Use of Abbreviation
A comment by user CauCuKien asks:
Can I use WA instead "WhatsApp" on my app title like "Auto Reply for WA"?
Since in such a case "WA" would be intended to be understood as meaning "WhatsApp", the rules would be much the same as if the app caption were to be "Auto Reply for WhatsApp". That is, one may, in general, use a trademark, or an abbreviation of that mark to indicate that a product is compatible with another product, or is intended to be used with that other product.. A product might, for example, be called "RedyReply for Windows" to indicate that it is designed to be used on the Windows OS, without obtaining permission from Microsoft. But the maker must be careful not to say, imply, or give the impression that the product is endorsed, approved, or sponsored by the owner of the trademark, or is made by the same person or people or firm who made the trademarked product.
It is good practice to include a disclaimer, making this clear. Something like:
RedyReply is designed for use on Windows versions 9, 10, and 11. It has been tested and works well on those systems, but it is not officially approved or sponsored by Microsoft, the owner of the trademark on "Windows".
By including such a disclaimer, the maker of the (fictional) "RedyReply" does not pretend to a relationship with Microsoft that does not exist. Similarly, an app with a display title "Auto Reply for WhatsApp" would be wise to include a disclaimer or statement making it clear who owns the mark "WhatsApp" and that the app is not officially recognized, approved, or sponsored by the mark owner. The same would be true if the display used "WA" rather than "WhatsApp". Using the abbreviation does not make that much difference.
Such use of a mark, or its abbreviated form, is nominative use and is specifically allowed by US trademark law, and is permitted under the trademark law of all other countries that I know of, although I have not checked the law of all the many countries in the world. But it is no longer nominative use if a reasonable person would be led to believe that the product is sponsored, endorsed, or approved by the owner of the mark, or comes from the same source.