I’d bet if you looked at manuals they’d say “use on a hard flat surface.” Apple refers to them as “portables” rather than “laptops” for this exact reason, so when customers talk about a machine getting hot on their lap, the response can be “use it on a table or a desk to allow proper airflow.

MacBooks don’t have any vents on the bottom, so it seems it would actually be safe to use them on your lap.