Microsoft rolled out the Windows 10 Mobile 10586.29 update earlier this month, and while a majority of Lumia 950 and Lumia 950 XL owners were able to install the update without any hitches, a subset of Lumia 950 users were seeing low storage notifications, and had the update stuck at zero percent.
Microsoft has mentioned that it is actively investigating the issue, and until such time it rolls out a fix, the 10586.29 update is being pulled from public servers:
We are aware of issues related to the recently published Windows 10 Mobile update, including: update not being offered, update appearing to get stuck at 0%, and pesky low storage notifications. The team is working on fixes and improvements to the upgrade experience in the short term and with future releases. In the meantime, we have unpublished the 10586.29 update for Windows 10 Mobile from our public update servers while we work through these reports.
For those of you who still see the low storage notification: while bothersome you can safely ignore this message.
For those of you who still have pending updates (OS or apps): make sure to plug your phone into a charger and leave untouched while it works through those items.
Microsoft mentions that users that have already installed the update will not be impacted, and that they will be eligible to receive cumulative updates in the future.
Source: Microsoft Answers forum