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
bool if_exists = false;
    
    bool if_not_exists = false;
    
    ASTPtr order_by = nullptr;
    
    ASTPtr sample_by = nullptr;
    
    ASTPtr index_decl = nullptr;
    String after_index_name;
    
    String index_name;
    
    ASTPtr constraint_decl = nullptr;
    
    String constraint_name;
    
    ASTPtr projection_decl = nullptr;
    String after_projection_name;
    
    String projection_name;
    
    ASTPtr ttl = nullptr;
    
    bool ignore = false;
    
    bool clear = false;
    
    ASTPtr codec = nullptr;