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 operator()(const MergeTreePartInfo & lhs, const DataPartPtr & rhs) const { return lhs < rhs->info; }
        bool operator()(const DataPartPtr & lhs, const DataPartPtr & rhs) const { return lhs->info < rhs->info; }
        bool operator()(const MergeTreePartInfo & lhs, const PartitionID & rhs) const { return lhs.partition_id < rhs.toUnderType(); }
        bool operator()(const PartitionID & lhs, const MergeTreePartInfo & rhs) const { return lhs.toUnderType() < rhs.partition_id; }
    };
    struct LessStateDataPart
    {
        using is_transparent = void;
        bool operator() (const DataPartStateAndInfo & lhs, const DataPartStateAndInfo & rhs) const
        {
            return std::forward_as_tuple(static_cast<UInt8>(lhs.state), lhs.info)
                   < std::forward_as_tuple(static_cast<UInt8>(rhs.state), rhs.info);
        }
        bool operator() (DataPartStateAndInfo info, const DataPartState & state) const
        {
            return static_cast<size_t>(info.state) < static_cast<size_t>(state);
        }
        bool operator() (const DataPartState & state, DataPartStateAndInfo info) const
        {
            return static_cast<size_t>(state) < static_cast<size_t>(info.state);
        }
        bool operator() (const DataPartStateAndInfo & lhs, const DataPartStateAndPartitionID & rhs) const
        {
            return std::forward_as_tuple(static_cast<UInt8>(lhs.state), lhs.info.partition_id)
                   < std::forward_as_tuple(static_cast<UInt8>(rhs.state), rhs.partition_id);
        }
        bool operator() (const DataPartStateAndPartitionID & lhs, const DataPartStateAndInfo & rhs) const