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
{
        return storage_settings.get();
    }
    String getRelativeDataPath() const { return relative_data_path; }
    
    String getFullPathOnDisk(const DiskPtr & disk) const;
    
    
    DiskPtr tryGetDiskForDetachedPart(const String & part_name) const;
    DiskPtr getDiskForDetachedPart(const String & part_name) const;
    bool storesDataOnDisk() const override { return true; }
    Strings getDataPaths() const override;
    
    ReservationPtr reserveSpace(UInt64 expected_size) const;
    
    static ReservationPtr reserveSpace(UInt64 expected_size, SpacePtr space);
    static ReservationPtr tryReserveSpace(UInt64 expected_size, SpacePtr space);
    
    ReservationPtr reserveSpacePreferringTTLRules(
        const StorageMetadataPtr & metadata_snapshot,
        UInt64 expected_size,
        const IMergeTreeDataPart::TTLInfos & ttl_infos,
        time_t time_of_move,
        size_t min_volume_index = 0,