Many services won't need an RDBMS, and for those that do there's a difference between the control plane of an application (development and releases), and the data plane (users using the service).
This is a complex and nuanced topic, but on my previous team of ~6 where we built a custom solution, we decided against using an RDBMS for multiple reasons, and on my current team where we use the same flagging system across 15 or so >1m requests per second services, there's no way it would work for us. If it works for your use case, that's great! But my advice for anyone else reading would be to put a lot of effort into considering the options as it's hard to change later and has significant impact on how the flagging system is used.
As for whether to use a commercial platform... my preference is probably to build my own with what I need in a system that I can modify as needed, or a commercial platform if there's one ready to go at a good price with the right feature set. I probably wouldn't use an existing open source option here unless I was forking it and treating it as my own from then on, as I find these things need flexibility and customisation. I've yet to see a great open source option.
This is a complex and nuanced topic, but on my previous team of ~6 where we built a custom solution, we decided against using an RDBMS for multiple reasons, and on my current team where we use the same flagging system across 15 or so >1m requests per second services, there's no way it would work for us. If it works for your use case, that's great! But my advice for anyone else reading would be to put a lot of effort into considering the options as it's hard to change later and has significant impact on how the flagging system is used.
As for whether to use a commercial platform... my preference is probably to build my own with what I need in a system that I can modify as needed, or a commercial platform if there's one ready to go at a good price with the right feature set. I probably wouldn't use an existing open source option here unless I was forking it and treating it as my own from then on, as I find these things need flexibility and customisation. I've yet to see a great open source option.