It is a hard concept that isn't completely solved by any of the standard GUI widgets.
An input box with pill-style auto-completion (see e.g. the tags control on Stackoverflow) would be a reasonable alternative.
Another, larger alternative is the typical two-lists control, with selected items moved from the source list to the target list; this idiom was widely used in Windows since 1990s. It's not mobile-friendly though.
If you actually need to multi-select over 200 arbitrary values (you almost certainly don't), the best way to do it I've seen is a way to filter the displayed items, a "Select All" button for what's visible based on that filter, and maintaining the "checked" state through filter changes, allowing you to select anything matching multiple filters.
The only way to make a 200 item select box usable is to make it not a 200 item select box somehow.
I find that I often have half second hangups due to not remembering if in select mode or not.
Much prefer the "Hold shift/ctrl" model used often in file managers.
A multi-select drop down is just solving the problem of a checkbox list being too long, or not what the designer wants it to look like. The most clear UI for this input type is a set of checkboxes in my opinion. Same for a single item select and radio buttons.
Drop downs have a lot of caveats, and unique compositions in the wild, so many of them are inaccesible and it's essentially just because someone wanted it to look pretty. I think that is bad software.
Depending on the application, sometimes to fix such problems, you need to reimplement the component in js or you start won't fix'ing tickets.
So articles like this really underestimate the problems with the existing controls and the lack of some.
