@Shirly10 I've had guests like that- long standing profile and no reviews. In all those cases, it turned out to be totally innocent. One had joined because they were planning to travel, but never did, one had had 2 other stays but their hosts didn't bother to leave a review, one had stayed at many Airbnbs, but with her husband, booked under his account. She said she liked having her own account, because she enjoyed perusing listings all over the world, like you'd thumb through a travel magazine.
In all those cases, the guests sent me thoughtful, informative messages along with their requests, answered my messages right away- there were no red flags. And they all turned out to be great guests.
Nothing wrong with asking somewhere in your return message- "I see you have had an Airbnb account since 2016, but no reviews. I'm curious as to why that might be?"