How can I include the "Thank you Sent Date" filed in a report
Indeed, I am not seeing the "Thank you date" field as an option to be included in the built in reports, but you could do a find on contributions - if you export the primary fields, the thank you date ends up around column CC (might depend on what version of Civi you are using), or you can and select the relevant fields for export, the thank you date being one of them.
Hope this helps, Tamar
I had a client ask me about this recently, so I submitted a patch to add the thank-you dates to the Contribution Detail and Summary reports. You can patch CiviCRM now; it will likely be part of CiviCRM 5.12 or 5.13.