Complete rubbish is the best I can say about that. Mostly stories promoted by folk that sell Microsoft products and do not like the Open Source concept for their business profits I would think. No money to be made selling Thunderbird as it's use is free and unrestricted.

A decade ago Mozilla dumped Thunderbird and a new company was formed to run the development of Thunderbird funded by donations from Thunderbird users. Whilst ever folk keep giving the program will continue in development last year an Android version was developed on the bones of the K9 mail app. I understand the intention is for an iPhone version next year, so I would say Thunderbird is growing year on year, not facing extinction.