I have also seen the same alert when building a new campaign this week. I just spoke to a Klaviyo agent who advised me of the following:
"This alert is due to "Shop Now" being the default text on the block when it is brought in, and since this is the default configuration, the alert is to remind you that you may want to change it. With that said, this is a warning about that, and not something which would prevent you from sending the campaign. If you are good with that button text, then you can proceed in sending/scheduling out the campaign."
"There is no need to change anything, Our team is also working to remove that warning, but you are all good to send your campaign
"️