MongoDB has announced end of life (EOL) support for MongoDB version 3.4 in January 2020. This will impact all installations of Sitecore on version 8.2.x using MongoDB 3.4.
When a MongoDB EOL date is reached, you can no longer receive any support from MongoDB for that version of the software. Most organizations have policies about operating unsupported software in production environments.
MongoDB's policy page itself does not specify if this is an inclusive date for the whole month, but after reaching out to them Sitecore has confirmed that this is targeted for January 31st, 2020.
Sitecore’s product support lifecycle defines 3 levels of support: Mainstream, Extended, and Sustaining. You can consult the KB article on product support lifecycle to see what happens when you reach the end of Mainstream support, and also what dates each version will reach that stage: https://kb.sitecore.net/articles/641167
Version 8.2.7 (also known as 8.2 update 7) is available from dev.sitecore.net at: https://dev.sitecore.net/Downloads/Sitecore_Experience_Platform/82/Sitecore_Experience_Platform_82_Update7.aspx
That is a good question. If you are using MongoDB Atlas fully managed clusters, those existing clusters running 3.4 will be automatically upgraded to 3.6 on January 31, 2020. This means your Sitecore 8.2 installation will have incorrect drivers to connect to the hosted MongoDB.If you are self-hosting MongoDB, it will not suddenly stop working on that date, but it is no longer supported by the vendor. In some cases, you might be in a situation where it is acceptable to continue running unsupported software. For example, you may have an agreement with a third-party managed services group to provide full support for your MongoDB infrastructure. Often those third-party services groups themselves rely on support from the software vendor and may need to upgrade you to maintain that support. If you are in that type of services agreement, you should contact your service vendor and inquire if the EOL will have any impact to continued support.
If you do not have a group that is responsible for providing support, and do not upgrade to a supported version, then you will now have to take on full support of that product into your own team. Any issues that arise cannot be addressed by the vendor’s support team and you will need to rely on your own internal capabilities.
Sitecore Product Support Lifecycle: https://kb.sitecore.net/articles/641167