I don’t think you should follow developers, but projects.

If you are interested in back-end development in general, you are in trouble. I can give you a list of about 15 back-end framework in multiple languages off the top of my head, and if you start following all their core contributors, you are facing a list of at least 60 people.

What you should do instead is

• pick the language(s) of your choice
• look around the Interwebz, collect the most renown frameworks, but not more than two. With some languages you will be lucky as there is only one or two such frameworks, but if you take Python, for example, you have at least 4 to choose from. To aid you in picking the right frameworks, look at projects you are interested in, and check what frameworks they use.
• look at the repository of these frameworks, and find the most active contributors
• last, but most important, check out the profile of these people. If they have nothing in there but the framework you find, it probably doesn’t pay to follow them as all their contributions go towards that framework

At the end, you will have a bunch of people with interesting side projects. It worths noting that many of such side projects will not be related to back-end development, or even web development at all. Taking myself as an example, if you look at my GitHub and GitLab accounts, you won’t find too many web-related repos, but they still might be interesting to you.

Depending on the language you are interested in, look into following core contributors of the most popular backend frameworks in that language.