Basically, nothing. Live Mail will still work but will become increasingly outdated. At present the big change is that WLM will no longer sync your contacts and calendar without a work round. Have a look at this.
Windows Mail is a poor replacement. You might in the end want to switch clients to something line Thunderbird which is compatible with windows 10.
Superuser 2017/18 Use Kudos to say thanks Mark answer as "helpful" only when the problem is solved Please don't send me private messages unless I ask you to. I do not work for VM. The advice I give is based on my best understanding of VM policy and practice. You rely on it at your own risk.
What I have done is use Win 10 mail for my Hotmail account as it syncs calendar & contacts & use Windows Live Mail for all my other accounts. WLMail will continue to work but wil not be updated, you may want to download the offline installer for future reinstallation as once the software become obsolete this month the web installer will disappear.
Virginia/Doctor9fan. If I have helped please give me Kudos.