The last page of the "KSP-1 Modding: Release" subforum with a KSP-1 mod marked as "1.12.*", which I have seen after getting through from page 1, was 85 of 168,
Total space occupied by the downloaded mods (including grabbed youtubes and imgurs), is ~150 GB (including ~59 GB of all Kopernicus mods and ~87 GB of others).
I expect, it will be ~200 GB when I finish.
Total space of the previously downloaded old mod versions ~80 GB (including numerous duplicates), but this is is without other old downloads on other external HDD drives, which are obviously obsolete and not needed anymore.
I mean, maybe this info is useful for some sociological purposes, idk.
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To save the mods and save the space, I extract all versions of same mod into one directory, and 7-zip them into one archive.
As they mostly consist of same files, the archive is several times smaller than if keep them in their original archives.
The latest version of every mod I also store in a separated archive as hot reserve.
I don't really need them/all of them, but what if I will need something later? Thus, I pack them instead of removing.
Though, I keep only last versions of the planet packs, because unlikely I can ever launch an old KSP release to open and old planet pack version.
Except the KittopiaTech planet packs, because they are history.
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Usually I use this great utility to search/remove the duplicates.
https://www.nirsoft.net/utils/search_my_files.html