NNS Update: Deprecating MergeMaturity

TLDR; MergeMaturity (a type of merge_neuron request) is being retired, and will return a GovernanceError, starting in about a month.

Deprecating MergeMaturity

Hello ICPeople.

As per the motion proposal “Compounding Maturity”, merging maturity has been replaced by staking maturity. As part of the transition, since January of 2023, when a user asks to merge maturity via a MergeMaturity action in a manage_neuron request, the maturity is being staked instead of minted into ICP. The changes were discussed in this forum post.

To complete the transition, and given that MergeMaturity redirects to StakeMaturity already, we plan to remove support for MergeMaturity actions.

After it is removed, it will return a GovernanceError indicating that the functionality is not available with that request, and that StakeMaturity is an equivalent call.

For some time after this change, if we find out that there is a compelling reason to keep forwarding available from MergeMaturity to StakeMaturity, this change can be reversed.

When it is safe to remove MergeMaturity from the API without breaking clients, we may completely remove MergeMaturity from the API, at least 6 months after it has been returning a GovernanceError.