Since you've landed on this page, I'm guessing that your WhatsApp media is taking up too much space on your smartphone, and probably have tried to move the app to your SD card but to no avail - because the option's not available on the application settings. 

And now you're desperate - trying to figure out how to free up space without deleting your WhatsApp media.

So, there's actually a way - and this method worked for me on my Samsung device. Unfortunately, I don't have access to other smartphones to test to see if the feature's available for other phones as well.  

I'm currently using a Samsung A7 (2017) which is operating on Oreo and it worked.

Basically, what you need to do is to use the 'Storage booster' on your phone. When I was running low on storage, Samsung automatically prompted me and suggested me to use it to free up the internal storage. But if you didn't get the prompt or notification, it's likely that you can't launch it even though you could see this under the list of Apps on your phone.

The solution is simple. Found out that you could download and use apps like Nova Launcher to get it started.

How it works:

1. Once you've launched the Nova Launcher, long-press on the home screen to select a widget.

2. Once you're done, click on the 'Activities' block on top. (Widgets > Activities)

3. Once you've got to 'Activities', look for 'Storage booster' and then tap once to view the list of items under it. When you see the 'Move to SD card', make sure you long-press it to launch it because if you just tap on it, nothing will happen. 

4. Once you've done that, you could easily move all your WhatsApp media and even Messenger media to your SD card in one single step. 

As the description explains, you can "save social network content to the SD card to free up device storage".

Once you've done that, you'll find that all your media files - photos, videos and such - are stored in your SD card now. However, if you're used to searching for media between a person or group by accessing the 'Media' tab on your WhatsApp chat, if won't be there anymore.

Your WhatsApp media is no longer stored in your phone's internal storage and instead have been transferred to another folder on your SD card and WhatsApp does not recognise this. But don't worry, everything's still there on your phone - but just they are just in another folder, in another space that doesn't affect your internal storage space.