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
class AlterCommands : public std::vector<AlterCommand>
{
private:
    bool prepared = false;
public:
    
    
    
    
    void validate(const StorageInMemoryMetadata & metadata, ContextPtr context) const;
    
    
    void prepare(const StorageInMemoryMetadata & metadata);
    
    
    void apply(StorageInMemoryMetadata & metadata, ContextPtr context) const;
    
    bool hasSettingsAlterCommand() const;
    
    bool isSettingsAlter() const;
    
    bool isCommentAlter() const;
    
    
    
    
    MutationCommands getMutationCommands(StorageInMemoryMetadata metadata, bool materialize_ttl, ContextPtr context) const;
};
}