I use the switch widget found here:
Currently working on version 8.5 and we had zero problems with it.
What subversion of the modeller are you using? Is it pre-8.4.1? In 8.4.1 the support for dojox libraries was included again (see release notes of 8.4.1 here), so if you are not using a version of the modeller >= 8.4.1 it may be worth looking into that as a first option.
Otherwise you may have to update the widget to use the new React libraries instead.
The boolean slider is too old and we didn't bother updating the codebase for any Mendix 6/7 deprecations.
I have retired the Boolean Slider widget in favor of the Switch widget.