Signal forks can have unexpected behaviours like retaining deleted messages and also they don’t get updated at the same rate that Signal get updated.

There are ways to save messages before they are deleted even if the stock app is used. Do not ever rely on this feature to work in a “save” way.

Every couple of years I hear a story about hackers disturbing signal with backdoors, which would be impossible or very hard to be done If they blocked third party clients. (Ex: 1)

That is a problem the users who prefer 3rd party clients have to deal with. Obviously if you care enough to not use the official build, you of cause have to take care of using a trustworthy source. That is not “your problem” though.

The amount of people who use third party Signal clients are very few anyway.

That sounds a lot like “I don’t use it, so none else needs it either” argument. In my opinion, none of your arguments above are a good reason to combat 3rd party clients.