How to retrieve deleted messages

You misunderstand what the switch is for. iCloud is not really a 'backup' for Messages..

Using iCloud for Messages is more for syncing.

If all you have is an iPhone and you don't care about syncing your messages with other devices (like an iPad or a Mac), you could probably just turn that switch off and then your iPhone backups would contain your messages.

About backups for iPhone, iPad, and iPod touch - Apple Support

Even then you need to understand that when your iPhone backs up, it replaces the previous backup. So if you wanted to recover a message after switching to this configuration from several months back, unless you specifically set aside a backup by archiving it, you still would not be able to recover your deleted messages.

In any case, deleting Messages that you might need one day will always be problematic because having to restore your entire iPhone to a previous backup just to get text messages back will not be a fun endeavor.

Depending on how you do it, the process of deleting text messages is a 3 or 4 step process. Some complain about the number of steps of course.. but this process should be ensuring that you actually want to delete the message(s).

So it would seem at the time you deleted these messages, you didn't really think they were that important?

go to Settings > [your name] > iCloud and check the switch position for Messages

If the switch is ON, then you cannot recover your deleted messages.

If the switch is OFF, then you might be able to recover your messages IF you have an iPhone backup that contains the messages you are trying to recover. In which case you'd need to restore your iPhone to that backup. In doing so, you'd likely lose some data between now and whenever your backup occurred.