Bootstrap Switch (https://www.abeautifulsite.net/bootstrap-4-switches) with calc() replaced by calculated values - using 14px as base font size. This allows the styles to work with IE9+ (which has a rendering bug related to flex and calc()).

bootstrap-switch.scss

//
// Switches for Bootstrap 4.
// https://www.abeautifulsite.net/bootstrap-4-switches
//
// - Fully customizable with Sass variables
// - No JavaScript required
// - Fully accessible

//
// IMPORTANT: These Sass variables are defined in Bootstrap's variables.scss. You should import that file first, then remove these.
//
$font-size-base: 14px;
$font-size-lg: 17.5px;
$font-size-sm: 12.25px;
$input-height: 38px;
$input-height-sm: 31px;
$input-height-lg: 48px;
$input-btn-focus-width: 3.2px;
$custom-control-indicator-bg: #dee2e6;
$custom-control-indicator-disabled-bg: #e9ecef;
$custom-control-description-disabled-color: #868e96;
$white: white;
$theme-colors: ( 'primary': #08d );

//
// These variables can be used to customize the switch component.
//
$switch-height: 30.4px !default;
$switch-height-sm: 24.8px !default;
$switch-height-lg: 38.4px !default;
$switch-border-radius: $switch-height !default;
$switch-bg: $custom-control-indicator-bg !default;
$switch-checked-bg: map-get($theme-colors, 'primary') !default;
$switch-disabled-bg: $custom-control-indicator-disabled-bg !default;
$switch-disabled-color: $custom-control-description-disabled-color !default;
$switch-thumb-bg: $white !default;
$switch-thumb-border-radius: 50% !default;
$switch-thumb-padding: 2px !default;
$switch-focus-box-shadow: 0 0 0 $input-btn-focus-width rgba(map-get($theme-colors, 'primary'), .25);
$switch-transition: .2s all !default;

.switch
{
    font-size: $font-size-base;
    position: relative;

    input
    {
        position: absolute;
        height: 1px;
        width: 1px;
        background: none;
        border: 0;
        clip: rect(0 0 0 0);
        clip-path: inset(50%);
        overflow: hidden;
        padding: 0;

        + label
        {
            position: relative;
            min-width: 60.8px;
            border-radius: $switch-border-radius;
            height: $switch-height;
            line-height: $switch-height;
            display: inline-block;
            cursor: pointer;
            outline: none;
            user-select: none;
            vertical-align: middle;
            text-indent: 68.8px;
        }

            + label::before,
            + label::after
            {
                content: '';
                position: absolute;
                top: 0;
                left: 0;
                width: 60.8px;
                bottom: 0;
                display: block;
            }

            + label::before
            {
                right: 0;
                background-color: $switch-bg;
                border-radius: $switch-border-radius;
                transition: $switch-transition;
            }

            + label::after
            {
                top: $switch-thumb-padding;
                left: $switch-thumb-padding;
                width: 26.4px;
                height: 26.4px;
                border-radius: $switch-thumb-border-radius;
                background-color: $switch-thumb-bg;
                transition: $switch-transition;
            }

        &:checked + label::before
        {
            background-color: $switch-checked-bg;
        }

        &:checked + label::after
        {
            margin-left: $switch-height;
        }

        &:focus + label::before
        {
            outline: none;
        }

        &:disabled + label
        {
            color: $switch-disabled-color;
            cursor: not-allowed;
        }

            &:disabled + label::before
            {
                background-color: $switch-disabled-bg;
            }
    }
    // Small variation
    &.switch-sm
    {
        font-size: $font-size-sm;

        input
        {
            + label
            {
                min-width: 49.6px;
                height: $switch-height-sm;
                line-height: $switch-height-sm;
                text-indent: 57.6px;
            }

                + label::before
                {
                    width: 49.6px;
                }

                + label::after
                {
                    width: 20.8px;
                    height: 20.8px;
                }

            &:checked + label::after
            {
                margin-left: $switch-height-sm;
            }
        }
    }
    // Large variation
    &.switch-lg
    {
        font-size: $font-size-lg;

        input
        {
            + label
            {
                min-width: 38.4px;
                height: $switch-height-lg;
                line-height: $switch-height-lg;
                text-indent: 46.4px;
            }

                + label::before
                {
                    width: 76.8px;
                }

                + label::after
                {
                    width: 46.4px;
                    height: 46.4px;
                }

            &:checked + label::after
            {
                margin-left: $switch-height-lg;
            }
        }
    }

    + .switch
    {
        margin-left: 1rem;
    }
}