Using joomla 3.10.3 and php 7.4.
We are using a t3 template which is based on "t3 base" ( not t3 base-bs3) which was derived from t3 blank.
Until t3 plugin version 2.7.7 the plugin used to override joomlas native mod_menu, which lead to plugins/system/t3/base/html/mod_menu/default.php being used to render the menu.
This isn't the case anymore with the new plugin version 3.
I've verified this by adding modifications to plugins/system/t3/base/html/mod_menu/default.php which do not show up as expected in the rendered page.
As a workarround I've copied plugins/system/t3/base/html/mod_menu over to our templates directory templates/ourtemplate/html which gave the desired result. But this of course isn't a not a solution.