@Dora31
If you look on www.gov.uk/standard-visitor-visa/documents-you-must-provide
it gives you more indepth information on visa application procedures as you describe.
The answer to your question is that an applicant should have to supply such a document ONLY if specifically asked
and if they do have a requirement to provide a receipt, an airbnb booking is perfectly valid.
Asides from that, it is probably good practice to let guests needing visa advise and guidence seek it from professional
bodies and not to get too involved yourself. @Oomesh-Kumarsingh0 reply here illustrates clearly why.