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
virtual bool supportsDeduplication() const { return false; }
    
    virtual bool noPushingToViews() const { return false; }
    
    
    
    virtual bool hasEvenlyDistributedRead() const { return false; }
    
    virtual bool supportsSubcolumns() const { return false; }
    
    
    
    virtual bool supportsTransactions() const { return false; }
    
    
    virtual bool supportsDynamicSubcolumns() const { return false; }
    
    
    virtual bool prefersLargeBlocks() const { return true; }
    
    virtual bool isSystemStorage() const { return false; }
    
    
    using ColumnSizeByName = std::unordered_map<std::string, ColumnSize>;
    virtual ColumnSizeByName getColumnSizes() const { return {}; }
    
    
    using IndexSizeByName = std::unordered_map<std::string, IndexSize>;
    virtual IndexSizeByName getSecondaryIndexSizes() const { return {}; }