In QGIS 3.12, from the View menu I go to Toolbars to toggle several of them on/off. However, when I toggle one (either way) I'm returned immediately to the previous screen. I've tried holding down the Control key but this action is ignored. What do I need to do?
When toggling on or off a Toolbar from the View --- > Toolbars will automatically close the menu after clicking. A quicker way to add/remove toolbars or panel is to click on the tool bars and a context menu will open with the tools/panels you want to toggle.
I do not know of any way to toggle on/off many at the same time.
You could also remove multiple toolbars using PyQGIS,the information is available here
I don't think this issue is related to QGIS itself, but it's related to PyQt. It has been always like that. Because the menus ("View > Toolbars" and the right click context menus) are only menus and a menu normally disappears after clicking its any item. Though, in this case, one can find it tedious.
I have added a feature request for this issue. However, this is not an important issue among others.