We have multiple products on the backend and some of the things have sales now and then as well as changes to many products descriptions like charts and links etc. Is there a plugin out there where it will add a new box under the description where we can enter custom text/links and be able to add the same to multiple products, this will make changing many products information so much easier as we will only have to change just the one box.
This is very custom requirement and I believe you won't be able to find an extension for this. I would recommend use product import feature for your requirement.
What you can do is come up with a csv file having SKU of all the product you want to update the description. Simply import the file and it will update the description of all those SKU in one import. You can update multiple attributes in one import if you want.
Here you can see how to import product data in Magento2 : http://docs.magento.com/m2/ce/user_guide/system/data-import.html
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I used Multi-editor function of Mag-manager module for similar purpose when had to add the same text to description for various products. Descriptions for products were different, but I add to add the same phrase at the bottom. I just had to select necessary goods and use Multi-editor to update descriptions massively.