This honestly depends on the type of government agency, and the job. I worked at the state workforce office for 8 years. For the past five years, the state of Texas has not given pay raises. There were opportunities for promotions, but, they were accompanied by having to move to a different office. Still in San Antonio, but clear across town. So, the added commute time, fuel costs, and wear and tear on my car did not make it "worth it."
When there are no pay raises, sometimes agencies take care of certain people by "Reclassifying" them. That is, they say that the work they are doing is beyond the scope of their job description, so they give them a different title with a different pay scale.
Promotions don't come all that often in some fields because the people that hold the positions plan to stay there. Forever. And ever. Older people, especially stay, for a variety of reasons. But, in some fields, there is a lot of turnover. Employees in technology can find better paying jobs in the private sector, for example.
State jobs with high turnover sometimes get paid more in an attempt to retain employees. Child Protective Services is an example. I know you are interested in corrections. I would expect they would have a similar problem. Because of turnover, there should be more opportunity for promotions. Also, if a department within an agency grows, there is opportunity for promotion.
Salary and promotion are interrelated ideas, but not the same thing. If you are on a classification chart, an entry level, you can move up in pay, without being promoted. The City of San Antonio implemented a "Step pay plan" where every year you get a pay raise, up to ten years, where you reach your maximum salary. Then all you get is cost of living raises. https://www.sanantonio.gov/Portals/0/Files/EmployeeInformation/Compensation/PayRanges.pdf
I imagine corrections officers (state) would have steps, as well as promotions to Corrections Officer 2, Corrections Officer 3, Corporal, Sgt. etc. Promotions usually mean that you have increased responsibilities.
So basically, it depends on the agency and the job. It's not a sure thing. If you are definitely interested in corrections/law enforcement, I encourage you to consider the military. The training is good, the promotions are there, and, when you get out, it gives you an advantage in other jobs.
Hope this helps!
However, I recommend continually reflecting on where you want to go with your career and setting goals to get there. This will help ensure you receive the promotions you deserve! Within the government or outside of it, career growth and trajectory will require a little flexibility and attention.
Hope that's helpful.