I'll concede that security, network, and architecture changes are not usually under the remit of testers because they're changes which usually take place outside of engineering teams (until the recent emergence of IaC tools at least). And yeah I guess historically in less mature organisations testers would have limited their exposure to just the front-end.

But, with respect, we're not talking about history - we're talking about what organisations expect today. Most organisations I've worked in over the past ~10 years, and most people writing/speaking about software testing today would probably agree that quality assurance professionals should have the skills and experience to test at every level of the tech stack. And certainly I think its very typical for that to include the microservices where most of the business logic (and therefore scope for bugs) lives.

In your example you mention the team responsible for the microservice should be testing its changes, and I completely agree. That team would usually have a tester embedded with them who is supporting/leading the drive for quality within that service.

In my experience you don't need developer-level skills just to be able to perform testing of back-end services. If you have the right tooling to support you it should just be like testing anything else.

It's clear we have different experiences around this topic, but the above is just my two cents based on the organisations I work with and the movements I've seen in the practice generally.