Hi MMR443,
Existing contacts can resubscribe via the link of previous delivered email, because the link contains special parameters to identify them, so we can add notification text in the content of email and won't need to create other customer journeys again. No matter which email, what contacts can see in the subscription center are always same, the link will not expire if we keep subscription center marketing page live.
Set up a subscription center (Dynamics 365 Marketing) | Microsoft Docs
Below is description that how does SC works:
Subscription centers only work when they "know" who they are talking to. This enables the subscription center to display existing contact information in editable fields (like name and email), and also to indicate which of the available subscription lists the viewer already belongs to. The only way most people will be able to open the subscription center is by selecting a link sent to them in a marketing email from a customer journey. Links such as these include an ID that lets the subscription center know which contact has requested the page. (In fact, all links in marketing emails include an ID that's linked to both the contact and the message, which enables the system to report which link each contact has selected in each message.)
And in this section, you can see why transactional messages do not require subscription center link:
Set email options on the Summary tab for sender, receiver, language, and designation (Dynamics 365 Marketing) | Microsoft Docs
Transactional messages are typically regulated differently from commercial messages. They are considered personal communications, not promotional communications, and therefore have different content and consent requirements. In Dynamics 365 Marketing, the main practical different between transactional and commercial messages is that different validation rules apply. Specifically, commercial messages require a subscription-center link while transactional messages do not. Also, Marketing manages consent independently for each of these two types of messages.
Regards,
Clofly
