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
DataPartPtr & out_covering_part,
        DataPartsLock & data_parts_lock) const;
    
    bool isPrimaryOrMinMaxKeyColumnPossiblyWrappedInFunctions(const ASTPtr & node, const StorageMetadataPtr & metadata_snapshot) const;
    
    using MatcherFn = std::function<bool(const String &)>;
    PartitionCommandsResultInfo freezePartitionsByMatcher(MatcherFn matcher, const StorageMetadataPtr & metadata_snapshot, const String & with_name, ContextPtr context);
    PartitionCommandsResultInfo unfreezePartitionsByMatcher(MatcherFn matcher, const String & backup_name, ContextPtr context);
    
    bool canReplacePartition(const DataPartPtr & src_part) const;
    
    virtual void dropPartNoWaitNoThrow(const String & part_name) = 0;
    virtual void dropPart(const String & part_name, bool detach, ContextPtr context) = 0;
    virtual void dropPartition(const ASTPtr & partition, bool detach, ContextPtr context) = 0;
    virtual PartitionCommandsResultInfo attachPartition(const ASTPtr & partition, const StorageMetadataPtr & metadata_snapshot, bool part, ContextPtr context) = 0;
    virtual void replacePartitionFrom(const StoragePtr & source_table, const ASTPtr & partition, bool replace, ContextPtr context) = 0;
    virtual void movePartitionToTable(const StoragePtr & dest_table, const ASTPtr & partition, ContextPtr context) = 0;
    virtual void fetchPartition(
        const ASTPtr & partition,
        const StorageMetadataPtr & metadata_snapshot,
        const String & from,
        bool fetch_part,
        ContextPtr query_context);
    virtual void movePartitionToShard(const ASTPtr & partition, bool move_part, const String & to, ContextPtr query_context);
    void writePartLog(
        PartLogElement::Type type,
        const ExecutionStatus & execution_status,
        UInt64 elapsed_ns,
        const String & new_part_name,
        const DataPartPtr & result_part,