Now it support only `String` and `FixedString`

https://github.com/ClickHouse/ClickHouse/blob/33aefa832b3c544934a757cc9d0e284252a1dd3e/src/Storages/MergeTree/MergeTreeIndexFullText.cpp#L809

Seems that `LowCardinality(String)` can be supported too, because we only use `column->getDataAt` to retrieve data and it will work on `LowCardinality(String)` in same way as for `String`
https://github.com/ClickHouse/ClickHouse/blob/33aefa832b3c544934a757cc9d0e284252a1dd3e/src/Storages/MergeTree/MergeTreeIndexFullText.cpp#L543
https://github.com/ClickHouse/ClickHouse/blob/33aefa832b3c544934a757cc9d0e284252a1dd3e/src/Storages/MergeTree/MergeTreeIndexFullText.cpp#L145
{
            /** Try to parse arrayElement for mapKeys index.
              * It is important to ignore keys like column_map['Key'] = '' because if key does not exists in map
              * we return default value for arrayElement.
              *
              * We cannot skip keys that does not exist in map if comparison is with default type value because
              * that way we skip necessary granules where map key does not exists.
              */
            if (value_field == value_type->getDefault())
                return false;
            const auto * column_ast_identifier = function->arguments.get()->children[0].get()->as<ASTIdentifier>();
            if (!column_ast_identifier)
                return false;
            const auto & map_column_name = column_ast_identifier->name();
            size_t map_keys_key_column_num = 0;
            auto map_keys_index_column_name = fmt::format("mapKeys({})", map_column_name);
            bool map_keys_exists = getKey(map_keys_index_column_name, map_keys_key_column_num);
            size_t map_values_key_column_num = 0;
            auto map_values_index_column_name = fmt::format("mapValues({})", map_column_name);
            bool map_values_exists = getKey(map_values_index_column_name, map_values_key_column_num);
            if (map_keys_exists)
            {
                auto & argument = function->arguments.get()->children[1];
                if (const auto * literal = argument->as<ASTLiteral>())
                {
                    auto element_key = literal->value;
                    const_value = element_key;
                    key_column_num = map_keys_key_column_num;
                    key_exists = true;
                }