As footnote 1 in the featured article mentions, Redis is not open source anymore. From Redis [1]:

Redis is source-available software, available under both the Redis Source Available License v2 (RSALv2) and the Server Side Public License v1 (SSPLv1).

Redis Stack and all Redis modules created by Redis Ltd. (e.g., RediSearch, RedisJSON, RedisGraph, RedisTimeSeries, and RedisBloom) are dual-licensed under the Redis Source Available License v2 (RSALv2) and SSPL.

Redis Enterprise is closed source and requires a commercial license from Redis Ltd.

There are previous versions of Redis under the 3-clause BSD license (free and open source [2]) [1]:

Can I continue to use versions of the products that were provided under the original 3-clause BSD license?

Yes. The license change is not retroactive. This means all source code and releases prior to the change remain under the 3-clause BSD license. You may continue to use those versions indefinitely under the original license, as long as you abide by its terms and conditions.

[1] https://redis.com/legal/licenses/

[2] https://en.wikipedia.org/wiki/BSD_licenses#3-clause_license_...