With all necessary respect for both "sides" here, in my opinion the reverting of this feature was the right decision.

Also, stuffing the Customizer with even more features in this tiny little area is a thing that should be debated more in general.

When the theme search isn't working any good on WordPress.org at all, why should this even land in the Customizer. Just makes no sense.

For years, it is not possible to search for specific Child Themes for example. If you want ALL Child Themes in the repo for, let's say, Twenty Sixteen, you won't find them. You only will find those, that have the term "Child Theme" somewhere in their (child) theme description field. But a lot of authors forget that or whatever.

Please please, fix the search and filtering of themes first, and then discuss any features to bring theme search/filtering to more places into WordPress core.

And if fixing the filter function is not possible or not wanted then please give a full text search that would be worth this name!