The protocol only provides functionality to add yangs, but there is no corresponding method for removal. Furthermore, the protocol allows to suspend yangs, but even if a yang is permanently  suspended it cannot be removed.
However, all yangs ever added (no matter if suspended) are iterated by the seer in order to query the orcale(s) for the underlying assets prices.  
Consider the following scenario:
A collateral asset is hacked and/or there is a scandal about the project team (for example, Terra Luna). Therefore, the asset crashes. It gets delisted from major exchanges and suspended from Opus.
Allow the protocols admin to remove yangs or at least exclude them from price updates. Furthermore, the 0 price assertion is dangerous, even during normal operation. Zero prices should be handled in a graceful way, especially in case of suspended assets, in order to avoid freezing the whole protocol.
