I've worked in organizations that promoted engineers into management, rather than bringing in outside managers or managers without engineering backgrounds. For highly engineering-driven orgs this can make a lot of sense, and depending on how it's done and the people involved, it can produce great results.

However: when those managers hold on to their engineering responsibilities, their teams suffer for it. There's a negative stereotype among engineers that managers don't do anything anyway, so maybe the team just needs a designated engineer to play manager in order to go to meetings, approve days off, etc.- the bookkeeping parts of management, but not the managing parts of management. There's a perception that the other engineers on the team are super self sufficient, what do they need a manager for anyway?

Often these managers would be allowed to continue doing engineering as a motivator- they didn't really want to be managers in the first place, let's just let them keep doing their thing and throw some more money at them and everybody will be happy. But it really feels like an antipattern.

Working on different teams where managers didn't do any coding was a breath of fresh air. I didn't know what I was missing. Managers can do the most good by focusing on things that allow their engineers to be as focused on their work as possible, and as unblocked and undistracted as possible. It makes a huge difference. It doesn't matter how incredibly self-sufficient an engineer is...if they get stuck doing a bunch of extra non-engineering stuff because their manager doesn't manage, their work and happiness will suffer for it.

So, at least based on my own experiences, I've come to see teams where the manager still has engineering responsibilities on their own team as a red flag. Anecdotal and highly contextual to where I've worked, but that's been my experience.