If your iPhone or iPad model supports Apple Intelligence and Apple Intelligence is turned on you can opt to summarise notifications from some or all of your apps. Actually, that isn't quite all notifications since Apple Intelligence isn't able to summarise some items, but it will probably summarise most of your notifications if you opt in. Your Inbox in the Mail app will also mostly give summaries of inbox messages in place of the first few lines of messages. I find the inbox summaries very useful to help me decide if I want to read a message now, wait until later or just delete it immediately. However, it's not always so convenient to hav a summary of a message or a WhatsApp chat instead of the full text on my lock screen. As a VoiceOver user I prefer to have the full message spoken to me when I tap it on the lock screen or in the notification centre. So how do you take control?
Summarisation for the Messages and Mail apps is treated differently from other apps. To control summarisation of Messages notifications, go to Settings / Apps / Messages / Summarise Messages. The setting for Mail is at Settings / Apps / Mail / Summarise Message Previews. I recommend that you have Mail previews summarisation turned on but you might want to have Messages summarisation turned off. For other apps, including WhatsApp, go to Settings / Notifications /Summarise Notifications.
If Summarise Notifications is turned off this may be your best choice. If you want to enable notifications summarisation for some or all of your apps, turn the switch on. You will then be presented with groups of apps for which you can control summarisation by group. At present, summarisations for the News and Entertainment group is temporarily disabled due to some disastrous errors in news story summarisations. If you turn this switch on, you'll start getting summaries when Apple believes it has fixed the problem. Once you select one or more groups of apps to summarise and confirm your choice you will be presented with the full list of your apps and can refine your selection and change summarisation settings for individual apps if you wish.