Right now clickhouse allows to create TTL columns for Distributed tables during alters. It is working normally until server restart. After that it won't start because engine distributed doesn't support TTL clause. Modify metadata is required.

My cluster has 21.3 but tested it also with 21.11 and the same behavior.
It is blocked during create but it should be also be blocked during alters.
Tested on table from tutorial
ENGINE = Distributed('stage, 'default', 'hits_v1') 
alter table hits_v1_all modify column UTMCampaign String TTL EventDate + INTERVAL 1 MONTH ← it passes normally
if (!alter_commands.empty())
    {
        
        for (const auto & command : alter_commands)
        {
            if (command.type != AlterCommand::MODIFY_DATABASE_SETTING)
                throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Unsupported alter type for database engines");
        }
        for (const auto & command : alter_commands)
        {
            if (!command.ignore)
            {
                if (command.type == AlterCommand::MODIFY_DATABASE_SETTING)
                    database->applySettingsChanges(command.settings_changes, getContext());
                else
                    throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Unsupported alter command");
            }
        }
    }
    return res;
}
AccessRightsElements InterpreterAlterQuery::getRequiredAccess() const
{
    AccessRightsElements required_access;
    const auto & alter = query_ptr->as<ASTAlterQuery &>();
    for (const auto & child : alter.command_list->children)
        boost::range::push_back(required_access, getRequiredAccessForCommand(child->as<ASTAlterCommand&>(), alter.getDatabase(), alter.getTable()));
    return required_access;
}
AccessRightsElements InterpreterAlterQuery::getRequiredAccessForCommand(const ASTAlterCommand & command, const String & database, const String & table)