This is essentially a duplicate of @2469
Except I'm clarifying that installing 2.8.x STM board package in a 1.8.x Arduino causes the Ports menu item to be grayed out in the tools menu FOR ALL BOARDS, not just when an ST board is selected.
Granted that ST Board Package 2.8.x is not supported except in Arduino IDE 2.x, it is still unacceptable that installing it would break all other platforms.
(I don't even understand how that's possible. Isn't the "Ports" menu computed entirely by the IDE, without involving platform code?)