As releases are usually bounded by e.g. development capacity, or certain themes (marketing message) it should be possible to close a release if capacity is reached.
Closing means that the "owner" of the release can indicate that no more features can be added to a release. This is especially useful if there is a direct connection with e.g. Jira.
We offer a wide range of customizations throughout the application, as it pertains to releases and features. As you may know, we also have 4 different tiers of user roles and permissions (product owners, contributors, reviewers and viewers). These can potentially be leveraged as reviewers and viewers cannot add/change features in a release.
That said, we have received much community feedback as we have created these permissions and we do not currently have plans to make changes in this specific area. We feel that this type of action is better communicated in an offline manner so that the team is aware that only Release Owners are "allowed" to adjust the features in a release. We hope you can understand.