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 (!all_columns.has(command.column_name))
            {
                if (!command.if_exists)
                {
                    String exception_message = fmt::format("Wrong column name. Cannot find column {} to comment", backQuote(command.column_name));
                    all_columns.appendHintsMessage(exception_message, command.column_name);
                    throw Exception(exception_message, ErrorCodes::NOT_FOUND_COLUMN_IN_BLOCK);
                }
            }
        }
        else if (command.type == AlterCommand::MODIFY_SETTING || command.type == AlterCommand::RESET_SETTING)
        {
            if (metadata.settings_changes == nullptr)
                throw Exception{"Cannot alter settings, because table engine doesn't support settings changes", ErrorCodes::BAD_ARGUMENTS};
        }
        else if (command.type == AlterCommand::RENAME_COLUMN)
        {
           for (size_t j = i + 1; j < size(); ++j)
           {
               auto next_command = (*this)[j];
               if (next_command.type == AlterCommand::RENAME_COLUMN)
               {
                   if (next_command.column_name == command.rename_to)
                       throw Exception{"Transitive renames in a single ALTER query are not allowed (don't make sense)",
                                                            ErrorCodes::NOT_IMPLEMENTED};
                   else if (next_command.column_name == command.column_name)
                       throw Exception{"Cannot rename column '" + backQuote(command.column_name)
                                           + "' to two different names in a single ALTER query",
                                       ErrorCodes::BAD_ARGUMENTS};