The new behavior you're seeing is due to the new modifyOtherKeys feature, which has applications such as Vim announce to the terminal that they would like to receive extended and expanded escape sequences for keypresses with modifier keys (Ctrl, Alt, Shift, etc.)
For example, this allows Vim to differentiate Ctrl+I from Tab, or Ctrl+[ from Esc, but it also allows Vim to recognize key sequences using both Ctrl and Shift modifiers together, for example, distinguish Ctrl+Shift+A from Ctrl+A, which has traditionally not been possible to do in a terminal.
As you've noticed, this also affects the Ctrl+Tab sequence, which the terminal starts passing to the application (in this case Vim) while the application has the modifyOtherKeys mode enabled.
Vim support for the modifyOtherKeys feature was introduced in Vim 8.2, so this seems to match your experience of having this behavior started around the time when you upgraded.
If you'd prefer to disable this feature, in order to keep your terminal processing the Ctrl+Tab sequence by switching terminal tabs, you can do so by adding these two lines to your vimrc:
let &t_TI = ""
let &t_TE = ""
You can find these instructions under
:help modifyOtherKeys. Note that these two lines disable this feature altogether, so you don't get all the other benefits of having Vim receive and be able to interpret an expanded list of key sequences.)
(If you'd like to keep this feature generally enabled, but have the terminal still handle the Ctrl+Tab events, you might want to consider filing an issue on mintty about perhaps adding an option for this, after all, the terminal could make it configurable which keys are passed to the application, even under modifyOtherKeys.)