If you do not currently have a job and want one, you can mark yourself with an Open to Work badge on Linked In. If you have a job, but are open to opportunities, there are ways to signal that without saying it outright.
The first thing you should do is make sure your current job and relevant ones are up-to-date. To show that this was a recent update, perhaps work in the month and year of a recent accomplishment. Generally, updating your resume is a signal that you are open to opportunities. I recommend customized resumes when applying to jobs, but that’s not possible on your Linked In profile, so customize it for the job you wish someone would approach you about.
Make sure your settings are set up so that you are accepting contacts, and that you are open to being contacted. This isn’t as visible as the green Open to Work badge, so it won’t draw attention from your current job, but it lets people know that you are ok with messages.
To draw some attention to your profile, you should have some activity. Reposts and comments are probably good enough.
Finally, it doesn’t hurt to expand your network. Visit profiles of people that might be interesting to work with and try to connect to them. You might find a chance to build a positive association with them over time. If you have people in your network that would write a recommendation, ask them for it.
Every time I talk to people that don’t use Linked In, they assume that you have to become a LI Influencer to get any attention, but for software developers, it’s enough to just be on it with a filled out profile, a decent network, and some light, but consistent activity. There are lots of people looking for you, and so you just need to make it look like you want to be contacted (if you do).