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
StorageFeatures features;
    };
    using Storages = std::unordered_map<std::string, Creator>;
    StoragePtr get(
        const ASTCreateQuery & query,
        const String & relative_data_path,
        ContextMutablePtr local_context,
        ContextMutablePtr context,
        const ColumnsDescription & columns,
        const ConstraintsDescription & constraints,
        bool has_force_restore_data_flag) const;
    
    
    void registerStorage(const std::string & name, CreatorFn creator_fn, StorageFeatures features = StorageFeatures{
        .supports_settings = false,
        .supports_skipping_indices = false,
        .supports_projections = false,
        .supports_sort_order = false,
        .supports_ttl = false,
        .supports_replication = false,
        .supports_deduplication = false,
        .supports_parallel_insert = false,
        .supports_schema_inference = false,
        .source_access_type = AccessType::NONE,
    });
    const Storages & getAllStorages() const
    {
        return storages;
    }
    std::vector<String> getAllRegisteredNames() const override