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
command.column_name = getIdentifierName(command_ast->column);
        const auto & ast_comment = command_ast->comment->as<ASTLiteral &>();
        command.comment = ast_comment.value.get<String>();
        command.if_exists = command_ast->if_exists;
        return command;
    }
    else if (command_ast->type == ASTAlterCommand::MODIFY_COMMENT)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.type = COMMENT_TABLE;
        const auto & ast_comment = command_ast->comment->as<ASTLiteral &>();
        command.comment = ast_comment.value.get<String>();
        return command;
    }
    else if (command_ast->type == ASTAlterCommand::MODIFY_ORDER_BY)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.type = AlterCommand::MODIFY_ORDER_BY;
        command.order_by = command_ast->order_by;
        return command;
    }
    else if (command_ast->type == ASTAlterCommand::MODIFY_SAMPLE_BY)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.type = AlterCommand::MODIFY_SAMPLE_BY;
        command.sample_by = command_ast->sample_by;
        return command;