You might not see this feature yet. Some functionality is introduced gradually to organizations that have set up the Targeted release options in Microsoft 365.
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Second, you mention that "pages and webparts are not supported at this time" but this documentation indicates that "Pages" is supported. can you please confirm?
Third, regarding the number of site designs supported in a tenant, some organizations were used to the concept of "Save as Site Template" to quickly create a new site based on that template. Some large organizations has different functional or regional areas where each area may required a list of Site Templates. These organizations may quickly hit this 100 limitation. Is there a reason why there is a limit of 100 site templates per tenant?
Fourth, if pages and webparts are not supported, what is the best way to create sites based off a site that has been customized for the organization? Say, the Sales team creates a site for a product and they want to create exact same site for other products, how would they handle this?
Finally, despite what others are saying about "save site as template" feature for modern sites, can you please confirm whether this is supported/possible for modern sites or not? I have not been successful with "save site as template" for modern sites.