> macOS will deprecate older Macs 6-7 years after their release.
This is substantially inaccurate. Current versions of macOS run on nearly all Apple systems from 2012 (8 years old), with the exception of some 2012 Mac Pros. The limiting factor in most cases is GPUs -- macOS 10.14 and later require some GPU capabilities which weren't reliably available in 2012.
Catalina, released in October 2019, dropped support for MacBooks released before 2015, MacBook Air models from before mid-2012, MacBook Pro models from before mid-2012, Mac Minis from before late 2012, and Mac Pros from before late 2013[1]. Do the math and that is 5 to 7 years between initial release of the hardware and deprecation by macOS.
> dropped support for MacBooks released before 2015
Those machines were all sold in 2011 or earlier. Saying "before 2015" is misleading, because the MacBook name was used during two disjoint periods to refer to two completely different machines.
Between 2006 and mid-2011, the MacBook brand name was used for a line of low-cost Core 2 laptops, most of which had plastic cases. (Some sales to schools continued through 2012.) These are the laptops which were not supported by macOS 10.14 and later.
Between mid-2011 and 2015, there were no computers sold under the MacBook brand. Apple only sold laptops under the MacBook Air and MacBook Pro brands during this period.
In 2015, Apple reused the MacBook brand name for a line of 12" ultraportable laptops. These are supported under current releases of macOS.
My point is that, in this context, "models released before 2015" really means "models released before 2012", because there were no MacBook computers on the market between 2012 and 2015. Using the phrasing "released before 2015" implies that there were some MacBooks from 2014 which Apple dropped support for, which is not the case.
The T2 chip can prevent people from putting their OS of choice on their hardware once Apple deprecates support for their machine.