.lib-url-check is undefined Magento 2.3 static content deploy error

Question

Magento 2.3 when deploying static content this error occurs:

Compilation from source: /var/www/html/MyProject/vendor/magento/theme-frontend-blank/web/css/styles-m.less .lib-url-check is undefined in /var/www/html/MyProject/var/view_preprocessed/pub/static/frontend/Magento/mytheme/nb_NO/css/source/lib/_icons.less in _icons.less in _module.less

And I believe due to the undefined mixin, styles-m.css is not generated as well prompting this:

The contents from the
"/var/www/html/MyProject/pub/static/frontend/Magento/mytheme/nb_NO/css/styles-m.css" file can't be read. Warning!file_get_contents(/var/www/html/MyProject/pub/static/frontend/Magento/mytheme/nb_NO/css/styles-m.css): failed to open stream: No such file or directory

pub/static has .htaccess so this is not the cause.
What else could it be, how to fix this?
My theme parent theme is blank.

UPDATE:
If anyone might get this error heres fix, in custom theme web/css/source/lib/_icons.less this code:

._lib-icon-image(
@_icon-image,
@_icon-image-width,
@_icon-image-height,
@_icon-image-margin,
@_icon-image-vertical-align,
@_icon-image-position-x,
@_icon-image-position-y
) {
.lib-url-check(@_icon-image);

.lib-css(background-image, @lib-url-check-output); // @lib-url-check-output is a returned variable of .lib-url-check() mixin
.lib-css(background-position-x, @_icon-image-position-x);
.lib-css(background-position-y, @_icon-image-position-y);
.lib-css(line-height, @_icon-image-height);
.lib-css(margin, @_icon-image-margin);
.lib-css(vertical-align, @_icon-image-vertical-align);
._lib-icon-image-size(
    @_icon-image-width,
    @_icon-image-height
);

Needs to be updated to:

._lib-icon-image-url(@_path) when not (@_path = false) {
.lib-css(background-image, url(@_path));
}

._lib-icon-image(
@_icon-image,
@_icon-image-width,
@_icon-image-height,
@_icon-image-margin,
@_icon-image-vertical-align,
@_icon-image-position-x,
@_icon-image-position-y
) {
._lib-icon-image-url(@_icon-image);
.lib-css(background-position-x, @_icon-image-position-x);
.lib-css(background-position-y, @_icon-image-position-y);
.lib-css(line-height, @_icon-image-height);
.lib-css(margin, @_icon-image-margin);
.lib-css(vertical-align, @_icon-image-vertical-align);
._lib-icon-image-size(
    @_icon-image-width,
    @_icon-image-height
);

After this static content compiles with no errors.

Answer 1

The lib-url-check mixin has been removed by the look of it, therefore you need to update your LESS files.

See this link for the commits - https://github.com/magento/magento2/search?p=1&q=lib-url-check&type=Commits