Hi!
We appreciate the feedback we have received from everyone, and we have discussed it internally.
The features we are removing are deprecated and should not have been used anymore. They all have alternatives that work equally well if not better.
For the authentication drivers, you can use passwd, pam and Lua as replacements for most of them. Lua in particular allows good integration with just about any external system. VPopmail can be replaced with SQL authentication.
For password schemes, we have guide: https://wiki2.dovecot.org/HowTo/ConvertPasswordSchemes
Memcached should be replaced with redis.
The expire, autocreate and autosubscribe plugins can be replaced with namespace settings:
namespace { mailbox Name { auto = create or subscribe autoexpunge = value } }
See the mailbox configuration documentation at https://doc.dovecot.org/configuration_manual/namespace/#mailbox-settings.
fts-squat can be replaced with Solr. squat has been considered obsolete (and that has been also indicated in documentation) since at least 2014.
After discussing it internally, we decided to postpone the xz removal for the time being. We understand the complexity of migrating away from it, so we want to give more time to do that. However beware that there are memory management issues in liblzma and we consider it unsafe to use. Feel free to use any of the other supported compresion algorithms instead. (We are also adding zstandard support in 2.3.11.)
You can switch your repository configuration to not use the ce-2.3-latest symlink, but rather to use a specific version (e.g., ce-2.3.10) giving you the control about when the system upgrades to a new version without missing out on CVE fixes in updated packages.
Finally, I want to point out that we will be happy if someone wants to start maintaining a feature we are planning to remove.
Aki