Microsoft has confirmed that Windows Phone 8.1 handsets with only 512MB of RAM will not be upgraded to Windows 10 Mobile. It was believed this was the case since only smartphones with 1GB of RAM are included in Microsoft's own eligibility list, which was published alongside the rollout of Windows 10 Mobile.
As spotted by WinBeta, to ensure there's absolutely no confusion, the official account for the Insider program confirmed this as the case on Twitter:
@coreyd_0512 We do not have plans to put W10M on devices with 512 MB ^TJ— Windows Insider (@windowsinsider) March 17, 2016
Microsoft previously spoke about how there could be potential limitations with 512MB with regards to Windows 10 Mobile, and while insider testers may have had a smooth enough experience on hardware with only 512MB to test the OS upgrade, Microsoft may have considered all potential issues and felt it necessary to remove said devices from the eligible list.