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
SpacePtr getDestinationForMoveTTL(const TTLDescription & move_ttl, bool is_insert = false) const;
    
    
    bool isPartInTTLDestination(const TTLDescription & ttl, const IMergeTreeDataPart & part) const;
    
    
    
    size_t getTotalMergesWithTTLInMergeList() const;
    using WriteAheadLogPtr = std::shared_ptr<MergeTreeWriteAheadLog>;
    WriteAheadLogPtr getWriteAheadLog();
    MergeTreeDataFormatVersion format_version;
    
    const MergingParams merging_params;
    bool is_custom_partitioned = false;
    
    Int64 minmax_idx_date_column_pos = -1; 
    Int64 minmax_idx_time_column_pos = -1; 
    
    static ExpressionActionsPtr getMinMaxExpr(const KeyDescription & partition_key, const ExpressionActionsSettings & settings);
    
    static Names getMinMaxColumnsNames(const KeyDescription & partition_key);
    
    static DataTypes getMinMaxColumnsTypes(const KeyDescription & partition_key);
    ExpressionActionsPtr getPrimaryKeyAndSkipIndicesExpression(const StorageMetadataPtr & metadata_snapshot) const;
    ExpressionActionsPtr getSortingKeyAndSkipIndicesExpression(const StorageMetadataPtr & metadata_snapshot) const;
    
    
    CompressionCodecPtr getCompressionCodecForPart(size_t part_size_compressed, const IMergeTreeDataPart::TTLInfos & ttl_infos, time_t current_time) const;
    std::lock_guard<std::mutex> getQueryIdSetLock() const { return std::lock_guard<std::mutex>(query_id_set_mutex); }