I did exactly same thing (add .nomedia on whatsapp xxxxx folder.)
But it still showing in gallery.
& there is no option to clear gallery data.
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For new photos / videos:
Whatsapp -> tap on 3-dot menu icon and select Settings -> Chats -> move the toggle next to Media Visibility to OFF position
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Quick and dirty
rename whatsapp_xxx_folder -> .whatsapp_xxx_folder
Whatsapp will create a new whatsapp_xxx_folder
move the toggle next to Media Visibility to OFF position
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.nomedia solution (not tested)
- copy ".nomedia" file from "/sdcard/Android/media/com.whatsapp/WhatsApp/Media/WhatsApp Voice Notes"
- paste it into "/sdcard/Android/media/com.whatsapp/WhatsApp/Media/YOUR_FOLDER"
clear the cache from Gallery
delete "/sdcard/Android/data/com.miui.gallery/files/gallery_disk_cache/small_size"
delete "/sdcard/Android/data/com.miui.gallery/files/gallery_disk_cache/full_size"
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