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 DB::extractKeyExpressionList(node);
    }
    /** Create local backup (snapshot) for parts with specified prefix.
      * Backup is created in directory clickhouse_dir/shadow/i/, where i - incremental number,
      *  or if 'with_name' is specified - backup is created in directory with specified name.
      */
    PartitionCommandsResultInfo freezePartition(
        const ASTPtr & partition,
        const StorageMetadataPtr & metadata_snapshot,
        const String & with_name,
        ContextPtr context,
        TableLockHolder & table_lock_holder);
    
    PartitionCommandsResultInfo freezeAll(
        const String & with_name,
        const StorageMetadataPtr & metadata_snapshot,
        ContextPtr context,
        TableLockHolder & table_lock_holder);
    
    PartitionCommandsResultInfo unfreezePartition(
        const ASTPtr & partition,
        const String & backup_name,
        ContextPtr context,
        TableLockHolder & table_lock_holder);