Online stores never have an “Exclude X” feature in the filter menus. E.g. I can filter to include all blue clothes but can’t exclude them.
From a query point of view this is not difficult. Would it be a UI nightmare? I don’t think so.

Not just clothing stores either: it's rare in general.

Feels like I only see it in detailed systems like dashboards for devs, analytics or business

It's a bit of a UI challenge.

There are many common solutions for inclusion.
Like a plain checkbox. Or a Multiselect Dropdown.
I can't think what a standard UI approach for excluding would be.

When I see it stuff like log dashboards, it's desktop adapted with "..."-buttons for handling include/exclude.

Sure, Plus & Minus button work. but in a multi select dropdown: each option being a fully clickable surface with a single toggle function is way cleaner. compared to having a label with two buttons. especially on mobile.

Allthough I could imagine a solution where you simply have two buttons: "include a filter" and "exclude a filter".

most clothing sites on mobile already basically have an "include a filter"-button that opens a menu with all their filters. so that wouldn't be simple and you wouldn't have to modify the UI of the actual filter select element other than a theme to indicate if you're in include-mode or exclude-mode